Wellsite Geology Manual - OMV

Procedures For Wellsite Geologists

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Compiled by Ian Willis September 2001

PREFACE This document is intended for the guidance of the Wellsite Geologist. The successful conduct will, require the application of initiative and professionalism consistent with the prevailing conditions. The principal purpose of this document, therefore, is to provide a reasonable and consistent approach to the conduct of field operations, not to dictate a rigid code of practice. The Procedures are compiled to provide an overview of the responsibilities of the Wellsite Geologist followed by chapters in which individual topics are discussed in more detail. It is hoped that persons who have direct experience in any of these topics will assist in improving the procedures by passing pertinent comments to the attention of the Operations Geologist.

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GEOLOGICAL WELLSITE PROCEDURES Table of Contents 1 PREAMBLE........................................................................................................................1 1.1 DUTIES AND RESPONSIBILITIES OF THE WELLSITE GEOLOGIST........................................................1 1.2 REPORTING FROM WELLSITE............................................................................................................................1 1.3 SAMPLE DESCRIPTION..........................................................................................................................................2 1.4 MUDLOGGING SUPERVISION..............................................................................................................................3 1.5 CORING.......................................................................................................................................................................3 1.6 WIRELINE LOGGING..............................................................................................................................................4 1.7 PRODUCTION TESTING/DRILL STEM TESTING............................................................................................5 1.8 LIAISON WITH WELLSITE MANAGER (DRILLING SUPERVISOR)...........................................................5 1.9 DATA DISTRIBUTION..............................................................................................................................................5 1.10 CONFIDENTIALITY...............................................................................................................................................5 1.11 DATA COMPILATIONS.........................................................................................................................................6 1.12 WELLSITE SUPPLIES............................................................................................................................................6

2 REPORTING PROCEDURES............................................................................................7 2.1 INTRODUCTION........................................................................................................................................................7 2.2 MORNING AND AFTERNOON DAILY REPORTS.............................................................................................7 2.3 DAILY REPORTING.................................................................................................................................................9

3 SAMPLE DESCRIPTION.................................................................................................12 3.1 INTRODUCTION......................................................................................................................................................12 3.2 DISCUSSION OF ELEMENTS COMPRISING A LITHOLOGICAL DESCRIPTION..................................12

4 SHOW EVALUATION......................................................................................................22 4.1 INTRODUCTION:....................................................................................................................................................22 4.2 REPORTING OF SHOWS:......................................................................................................................................22 4.3 SHOW EVALUATION FORM...............................................................................................................................23 4.4 MUD GAS SHOWS...................................................................................................................................................23

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4.5 ROUTINE HYDROCARBON DETECTION METHODS FOR CUTTINGS...................................................25 4.6 OTHER HYDROCARBON DETECTION METHODS FOR CUTTINGS.......................................................27 4.7 GENERALISATIONS...............................................................................................................................................29

5 DRILLING BREAKS, CORING AND SIDEWALL CORING............................................30 5.1 DRILLING BREAKS................................................................................................................................................30 5.2 CORING.....................................................................................................................................................................30 5.3 SIDEWALL CORING..............................................................................................................................................34

6 MUDLOGGING NOTES..................................................................................................36 6.1 INTRODUCTION......................................................................................................................................................36 6.2 LAG TIME.................................................................................................................................................................36 6.3 TOTAL GAS DETECTOR.......................................................................................................................................37 6.4 GAS CHROMATOGRAPH.....................................................................................................................................37 6.5 DRILL RATE.............................................................................................................................................................38 6.6 PIT LEVELS..............................................................................................................................................................38 6.7 OTHER COMMON EQUIPMENT........................................................................................................................38 6.8 SAMPLING PROCEDURES (CUTTINGS)...........................................................................................................38 6.9 EVALUATION OF MUDLOG SHOWS................................................................................................................39 6.10 FALSE HYDROCARBON SHOWS.....................................................................................................................40 6.11 PROBLEMS IN INTERPRETING CUTTINGS..................................................................................................41 6.12 MISCELLANEOUS INTERPRETATION PROBLEMS...................................................................................43

7 GEOCHEMICAL SAMPLING...........................................................................................45 7.1 INTRODUCTION......................................................................................................................................................45 7.2 CANNED CUTTINGS SAMPLES...........................................................................................................................45 7.3 MUD SAMPLES........................................................................................................................................................45 7.4 SIDEWALL CORES (SWC)....................................................................................................................................45 7.5 CORE CHIPS.............................................................................................................................................................45

8 GEOPRESSURES............................................................................................................46 8.1 ORIGIN OF ABNORMAL PRESSURES...............................................................................................................46

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8.2 DETECTION OF GEOPRESSURES DURING DRILLING...............................................................................47 8.3 PRESSURE DETECTION AFTER DRILLING....................................................................................................51

9 REPORTING FORMS......................................................................................................52 9.1 WELLSITE GEOLOGIST'S CHECKLIST...........................................................................................................53 9.2 DAILY GEOLOGICAL REPORT ........................................................................................................................56 9.3 CORE CHIP DESCRIPTION FORM.....................................................................................................................58 9.4 PERCUSSION SIDEWALL CORE DESCRIPTION............................................................................................60 9.5 ROTARY SIDEWALL CORE DESCRIPTION...................................................................................................61 9.6 HYDROCARBON SHOW EVALUATION FORM..............................................................................................62 9.7 WIRELINE LOGGING QUALITY CONTROL (with Diary and Horner Plot)...............................................63 9.8 MUDLOGGING QUALITY CONTROL CHECK LIST.....................................................................................64

APPENDICES.....................................................................................................................69 1 INTERPRETATION OF MUD GAS RATIO DATA...........................................................70 2 ROUNDNESS AND SPHERICITY, PERCENTAGE ESTIMATION.................................71 3 WELLSITE GEOLOGIST SUPPLIES INVENTORY........................................................72 4 CONVENTIONAL CORING..............................................................................................73 4.1 General........................................................................................................................................................................73 4.2 Coring Equipment......................................................................................................................................................73 4.3 SPECIAL CORING SYSTEMS...............................................................................................................................75 4.4 CORING BITS...........................................................................................................................................................78

5 DRILLING FLUID ENGINEERING...................................................................................81 5.1 INTRODUCTION......................................................................................................................................................81 5.2 HISTORY...................................................................................................................................................................81 5.3 FUNCTIONS OF A DRILLING FLUID.................................................................................................................82 5.4 DRILLING FLUID PROPERTIES.........................................................................................................................82 5.5 DRILLING FLUID ADDITIVES.............................................................................................................................88 5.6 DRILLING FLUID SYSTEMS................................................................................................................................91

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5.7 NEW DRILLING FLUID SYSTEMS.....................................................................................................................92

6 ABBREVIATIONS............................................................................................................96

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1 PREAMBLE 1.1

DUTIES AND RESPONSIBILITIES OF THE WELLSITE GEOLOGIST

The Wellsite Geologist is responsible for ensuring that geological data is collected, evaluated and recorded at the wellsite and reported to the Operations Geologist. These duties include supervision of the Mud Logging and Wireline Logging contractors and necessitate close liaison with the Wellsite Manager (Drilling Supervisor). The Wellsite Geologist’s Checklist (Section 9.1) is to be read and checked off as appropriate before leaving the office to travel to the rig. The Wellsite Geologist therefore has responsibilities that include those listed below. •

Observing and recording all hydrocarbon shows and evaluating their significance. •

Describing and recording (in the format defined in these Procedures) the lithological assemblages encountered in the well. It is important to emphasise that the Wellsite Geologist and not the Mudlogger has this function as his/her primary responsibility.

•

Witnessing and reporting wireline logging operations, ensuring adequate quality control.

•

Selecting core points based on Drilling Programme and Coring Criteria.

•

Submitting geological reports (morning and afternoon) to Operations Geologist consistent with the approved procedures.

•

Supervising the collection, packing and dispatch of ditch cuttings core samples and paper data from the rig. Although the Mudlogging Company performs these tasks, close attention to this important aspect of their work can save many hours of frustration and wasted effort later.

The Wellsite Geologist is empowered to implement the agreed formation evaluation programme, as defined in the Drilling Programme or subsequent modifications. Any deviations from that programme should be via a “Drilling Change Control Form Request”. Agreement to implement the proposed variance will be given by Drilling and Exploration. The Wellsite Manager (Drilling Supervisor) is responsible for the efficient, economic and safe performance of the drilling operation assuring suitable hole conditions for the well evaluation. It is important to keep the Wellsite Manager (Drilling Supervisor) informed of the expected evaluation programme, or of any programme changes, for his planning purposes. He is to be advised immediately of all hydrocarbon shows, increases in mud gas readings, significant changes in shale densities or any other factor which could effect rig safety or the proposed evaluation activities. All requests involving rig activity must be given through the Wellsite Manager (Drilling Supervisor) and not directly to drilling contractor personnel. 1.2

REPORTING FROM WELLSITE

It is important to maintain a regular and meaningful reporting procedure from the wellsite. Unless otherwise advised, the following reports should be made on a routine basis.

Title

Period 1

Time

Daily Geological Report Afternoon contact

Geological

Cuttings Description Report Wireline Logging QC and Diary and Horner Temp. Plot

Midnight-Midnight

Email by 07:30 with a verbal update

0600 – 16:00

Verbal contact, or short email about 16:00

While Drilling

Email daily

Completion of each logging Suite

Email

Core Chip Descriptions

As required

Email

Sidewall Core Descriptions

As required

Email

End of Hitch Report

Ongoing update of recommendations after each hitch

Email (even acknowledges quo”)

if it only the “status

Other reports and the distribution of data from wellsite is discussed in Section 2. Reporting Procedures. The format of these routine reports is described in Section 2. Reporting Procedures. Lithological descriptions for the Daily Reports are to be concise. Descriptions within a potential reservoir objective or pay zone, however, may be more detailed. In order to ensure uniformity of reporting from wellsite, it is imperative that the Wellsite Geologist co-ordinates with the Wellsite Manager (Drilling Supervisor) in all matters common to both the Geological and Drilling reports, before transmission.

1.3

SAMPLE DESCRIPTION

1.3.1

Introduction

The purpose of this procedural summary is to provide a methodology, which should result in a consistent and meaningful description of cutting samples within the constraints of working at wellsite. The preparation of cutting samples for examination is not addressed, as this subject is well documented in other references. Wellsite lithological descriptions are important in providing: • • •

An unambiguous interpretation of the lithological section drilled. A clear identification of reservoir facies and an assessment of their potential capacity to be hydrocarbon productive (made in association with hydrocarbon show data). A record of the degree of caving, contamination or other factors which may affect the utility of the final lithological log. 2

•

A product, which will allow for the compilation of a Composite Well Log when combined with wireline log data.

In order to accomplish these objectives, the following records are to be maintained. 1.3.2

Cuttings Description

The Wellsite Geologist should note the lithological and hydrocarbon show descriptions for each sample on the Cuttings Description Report. The Cuttings Description Report should accurately reflect the sample as observed by the Wellsite Geologist with the exception of obviously caved material. Separate notes should be made to reflect the Wellsite Geologist's interpretation of the significance of caved material, other contaminants and peculiarities. In order to provide uniformity of description from well to well and to avoid ambiguities resulting from differing styles adopted by geologists, the procedures for the description of cutting samples (Section 3. Sample Description) should be adopted. 1.3.2.1 Show Evaluation The Wellsite Geologist is responsible for the monitoring and reporting of all hydrocarbon shows from wellsite. In order to fulfil this function, the Wellsite Geologist must ensure that the Mudlogging Contractor is adequately prepared and maintains all equipment in a workable state (Section 6. Mudlogging). All Hydrocarbon Shows should be reported using the Show Evaluation Report, 9.6, as a guide.

1.4

MUDLOGGING SUPERVISION

The Wellsite Geologist is responsible for the direct supervision of the Mudlogging Contractor. The duties of the Mudlogging Contractor will be defined in Drilling Programme. Section 6. Mudlogging also contains information concerning the daily and other routine performance tests to be conducted on the Mudlogging Unit. Before going to wellsite it is important that the Wellsite Geologist is informed by the Operations Geologist the type of Mudlogging Unit in use and makes all reasonable attempts to ensure that he/she is familiar with the systems and procedures relevant, or peculiar, to that unit. The Wellsite Geologist must also check that adequate supplies of consumables are on the drilling unit upon his/her arrival.

1.5

CORING

The coring programme (where required), is an integral part of the Drilling Programme for each well. The Wellsite Geologist has the responsibility to ensure that any programmed cores are cut, 3

described and transported, following the procedures detailed in Section 5. Drilling Breaks, Coring and Sidewall Coring. Appendix 4 discusses the mechanical aspects of coring. Should unexpected circumstances occur which result in the Wellsite Geologist recommending an unprogrammed core, all relevant information must be transmitted to Operations Geologist with a recommendation, for review. Agreement to core will be made following consultation with others as may be required. Meanwhile, hole conditioning should be considered, in consultation with the Wellsite Manager (Drilling Supervisor). 1.6

WIRELINE LOGGING

The Wellsite Geologist is responsible for ensuring that the Wireline Logging Programme, as detailed in the Drilling Programme is completed in a competent and expeditious manner. The following general comments are relevant and are to be followed.. It is the responsibility of the Wellsite Geologist, to ensure that wireline logs of satisfactory quality are obtained. In order to accomplish this task, a series of quality control checks and procedures have been developed. During the logging job, the Wireline Logging Quality Control Report, Diary of Wireline Operations and an Extrapolated Temperature Plot (which are components of a linked set of spreadsheets) should be duly completed: 1.6.1

General Requirements

•

Ensure the Logging Engineer will be ready to start logging as soon as the drill pipe is out of the hole and the pre-logging job hazard analysis has been conducted.

•

Check with the Driller, Wellsite Manager (Drilling Supervisor) and Mudlogging Engineer about the condition of the hole, especially tight spots or bridges.

•

Provide a copy of the front page of the " Wireline Logging Quality Control Report " form to the Logging Engineer well in advance of the logging job. This will provide the engineer with coordinates, elevations etc. Discuss and clarify any queries.

•

Provide the Logging Engineer with any requested deviation data for the well (Totco or multishot).

•

Ensure that a circulated mud sample is collected and that resistivity measurements are made by the Logging Engineer for Rm, Rmc and Rmf. All resistivity data should be double checked for validity and all temperature data verified.

•

It is extremely important that a circulated mud sample be used for these measurements. Only in unusual circumstances should a pit sample be taken. If this is the case, the source of the sample should be annotated on the Log Header under "Remarks" as well as the appropriate "Source of Sample" box;

•

If a logging pill is spotted on bottom a sample of this should be collected from the appropriate pit and resistivity measurements made by the Logging Engineer for Rm, Rmc and Rmf. All resistivity data should be double checked for validity and all temperature data verified. This information should also be noted on the Log Header under “Remarks” as well as the appropriate "Source of Sample" box.

•

The Remarks section of Log Header should also include the following information; 4

% Barite in mud system, % Potassium in mud system, Base oil density in case of SBM mud systems, Origin of datum position (i.e. pip-tag, tide tables, etc), Time drilling ceased, Time circulation ceased, Time logging tool on bottom, Time logging completed, Average logging speed, Causes for any log anomalies, All thermometer readings. •

Ensure the Logging Engineer has all Header information correctly input.

•

Obtain a printed header sheet and proof read BEFORE final prints are made.

•

Ensure the Logging Engineer has instructions for the number of prints to be made.

•

Ensure a minimum of two thermometers are run on each tool string in every log suite where possible, including sidewall core runs.

1.7

PRODUCTION TESTING/DRILL STEM TESTING

Testing will be conducted consistent with the provisions of a Testing Programme which is subject to Joint Venturer and Government approval. It is unlikely that the Wellsite Geologist will be required to supervise or attend production testing. 1.8

LIAISON WITH WELLSITE MANAGER (DRILLING SUPERVISOR)

The Wellsite Geologist should keep the Wellsite Manager (Drilling Supervisor) informed of any changes in bulk lithology, reservoir objectives, potential lost circulation zones, potential overpressures and other matters which could reasonably be expected to influence the safety or efficient operations of the well. All depths and operations reported on the Geological Reports should be co-ordinated and agreed with the Wellsite Manager (Drilling Supervisor). 1.9

DATA DISTRIBUTION

The Wellsite Geologist is to ensure that all exploration data (reports, samples logs etc.) are correctly labelled, packaged and despatched in a timely manner. The timing and mode of shipment of data from the rig is included in the Drilling Programme. When in doubt, call the Operations Geologist It is extremely important that all data shipments from the rig be accompanied by a transmittal. A copy of the transmittal should be sent to Operations Geologist via email or fax. 1.10 CONFIDENTIALITY

5

All geological data should be considered as confidential. Such data should be discussed only with those persons directly involved in the use thereof (i.e. Mudloggers, Wellsite Manager (Drilling Supervisor)). Data necessary for the safe and efficient conduct of drilling operations should be provided to the Drilling Contractor Supervisor, the Drillers, the Mud Engineers and other relevant personnel, in co-ordination with the Wellsite Manager (Drilling Supervisor). Speculation upon the results of the well, and their significance, should be discouraged. 1.11 DATA COMPILATIONS The ultimate home for most of the data compiled at the wellsite is the GEOLOG data base. As this is an Excel based application Excel spread sheets should be used wherever possible to facilitate the loading of the data. 1.12 WELLSITE SUPPLIES A list of supplies that could be required at the wellsite are given in Section 3. Wellsite Geologist Supplies Inventory. The Wellsite Geologist must also ensure that the Mudlogging Contractor has sufficient consumables for the bagging and boxing of all samples and cores.

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2 REPORTING PROCEDURES 2.1

INTRODUCTION

Hard and fast rules on reporting are not appropriate to wellsite conditions. Consequently, the following text should be taken as a guideline. The rapid and accurate dissemination of data from wellsite is one of the most important tasks of the Wellsite Geologist and this function should be treated with care and thoroughness. 2.1.1

Routine Daily Reporting

Two reports on geological operations will be required from wellsite on a daily basis • A Morning Report is to be transmitted to Operations Geologist every day before 07:30, when a Wellsite Geologist is on location. Given that communication access is not unlimited, the most convenient time for transmission will be determined following discussions between the Wellsite Geologist, Drilling Supervisor and Operations geologist. This will determine the post midnight reporting period. • The informal afternoon report will be via telephone, or a short email message, about 16:00 when a Wellsite Geologist is on location. 2.2

MORNING AND AFTERNOON DAILY REPORTS

An example of the Word ® template used for the Morning Daily Geological Report is reproduced in Section 9.2. Instructions on the use of the report are given below, and any clarification may be obtained from the Operations Geologist. The Daily Report will be submitted by email. 2.2.1

Discussion

It is important to note that the effective time for the morning Daily Geological Report (DGR) is 0000 hours and will report all operations from the previous twenty four hours (i.e. 0000 - 2400). The file name of the file is to follow the convention: well name (eg Audacious-1), DGR (with report number), eg. georeport01, Date ( eg 28-01-98), midnight depth eg. 1505m: “Audacious-1_georeport01_28-01-98_1505m”. The Morning Report will contain the following information: Heading Data • • • •

This information (depth, progress, operation, deviation data, mud data etc.) should be in agreement with that reported by the Wellsite Manager (Drilling Supervisor). The Operations Summary should be just that - brief and to the point and should be confirmed with the Wellsite Manager (Drilling Supervisor). The Report Date is the date of the 24 hour period. The Report Number should be sequential by day, starting on the first full day of a Wellsite Geologist being on the rig. 7

• •

•

The midnight depth should always be confirmed with the Drilling Supervisor, and likewise other information such as water depth, RT elevation, casing depths, and FIT/LOT data. Mud data is obtained from the Daily Mud Report (usually via the Mudloggers) and ECD from the mudlogging Data Engineer. In addition, the Data Engineer should provide an estimate of the pore pressure. It is important to discuss the estimated pore pressure with the Data Engineer on a continual basis, as the safety of the well is potentially at stake. If the Dxc plot displays anything but a normal trend, then this should be discussed in the latter Formation Pressure Estimate part of the DGR. Survey data can be obtained from the MWD contractor, or from the Drilling Supervisor if MWD is not being run.

Lithology: •

•

• • •

The lithological descriptions for Daily Reports should be concise. Additional detail may be appropriate where the following circumstances arise:

• Significant shows are present, • Penetration of an important objective is imminent, • When approaching a core point, • Within the primary or other reservoir objective, • At the discretion of the Wellsite Geologist. The lithological section should be separated into intervals. Obvious interval breaks may occur at formation boundaries, and where there are significant lithological changes. Ultimately it is up to the common sense of the Wellsite Geologist as to where interval breaks are assigned, but intervals should be neither too fine nor too coarse. The ROP range and average are best derived by the Wellsite Geologist visually from the mudlog. An alternative method is for the interval penetration rate to be requested from the mudloggers. A brief summary of the interval lithology should precede the more detailed description. The detailed descriptions should be written in full (no abbreviations), and use the format described below (2.3.2)

Hydrocarbon Shows: •

Should be described using Section 9.6 Show Evaluation Form as a guide.

Gas Data: The following data will be obtained from the Mudlogging Engineer. • Background Gas Background gas intervals should be broken up on the basis of change in trends, and like lithological intervals, should not be broken up too finely or too coarsely. The best (only) way to pick intervals is graphically, from the mudlog. Once again, seeing things graphically gives the WSG a much better feel for what is happening. •

Trip Gas, Connection Gas and Gas peaks

To avoid confusion, trip gas, connection gas, and gas peaks should be absolute values, not values above background gas. Calcimetry 8

•

Calcimetry intervals should be broken up where there is a significant change in calcimetry, but neither too finely nor too coarsely. There is no point in continuing calcimetry measurements once ciral or similar CaCO3 lost circulation material has been added to the mud.

Formation Pressure Estimation •

Any departure from a normally pressured regime requires explanation. This should be done in conjunction with the mudlogging Data Engineer.

Sample Quality • •

Sometimes cuttings may be contaminated with cavings, cement, or mud additives, and these should be mentioned when in significant quantities. sample quality should be expressed as being either Unreliable, Questionable, or Good, with explanation required for the first 2 categories.

Mudlog Equipment/Personnel Detail any problems or rectification of problems. MWD •

Detail the sensor measuring points in metres behind the bit, list any equipment problems or changes, and include any pertinent remarks regarding log quality.

•

MWD Temperature

Safety: •

The Wellsite Geologist should make any relevant comments.

2.2.2 • • • •

2.3

Remarks

The remarks section is for any other pertinent information not mentioned elsewhere in the DGR, such as formation tops, comparison with the prognosis, further discussion of the significance of shows or any other matters of interest, and electric logging details. All transfers of materials from the rig should be noted here, with information concerning the mode of transport and ETA. If any significant changes in Mud Data have occurred within that period, it should be noted in the "Remarks" section. The WSG should always feel free to express any concerns or voice any opinions under this heading, as something the WSG may deem “un-newsworthy” may have later ramifications.

DAILY REPORTING

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2.3.1

Abbreviations

For the sake of consistency a list of abbreviations is attached as Appendix 6. Where the abbreviations are self explanatory they can be used in routine reports to Operations Geologist/Perth office with the exception of “Hydrocarbon Shows”. These should always be in longhand to avoid any misunderstanding. 2.3.2

Description Format for Daily Reports

The daily report descriptions of lithologies encountered will be extracted from the Cuttings Description Report and summarised in the order given in Section 3.1 and detail as indicated below. A typical lithological description is: 3250 - 3285 metres

Sandstone (100%), quartzose, silty, light olive grey, friable to moderately hard, fine to coarse, dominantly medium, fair to good sorting, subangular to subrounded, 20% quartz silt, trace clay matrix, 5-10% strong silica cement, trace glauconite and mica, fair intergranular visual porosity. Strong Show: 50% Bright yellow spotty fluorescence; instant streaming milky white cut; bright white residual fluorescence; thin straw coloured residual film. No odour Note: The percentage of the basic rock type is not written. Thus the 7075% quartz grains implied by the proportion of silt, mica and glauconite is implied but not stated.

2.3.3

Other Reports

A variety of other reports will be made to Operations Geologist from wellsite on an as-needed basis. These reports are included in the Section 9 Reporting Forms and listed in 1.2 Reporting from the Wellsite. The naming of digital report files is to be consistent follow a convention similar to that given below: Well name (eg Audacious-1), Type of data eg. MSCT Descriptions Suite 1, an appropriate depth or depth interval if applicable eg. 1505-1700m and file extension eg. .xls “Audacious-1_MSCT Descriptions Suite 1_1505-1700m.xls”.

2.3.4

Contractor Reports and Data

10

Many of the contractors provide reports and listings of ASCII data. The naming of their reports should follow a similar convention, for instance: Well name (eg Audacious-1), Type of data eg GR-DLL-AS-Cal_Suite-1 LAS Data (with run number or other descriptor if appropriate), an appropriate depth or depth interval if applicable eg. 1505-1700m and file extension eg. .las “Audacious-1_GR-DLL-AS-Cal_Suite-1 LAS_1505-1700m.las”.

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3 SAMPLE DESCRIPTION 3.1

INTRODUCTION

It is important to note that the following material is designed to address the description of cutting samples and not core samples. For all sample types however, the order in which the properties are to be described is given below. SAMPLE DESCRIPTION FORMAT ( in order) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Rock type (% and modifier, if required). Colour or colour range. Hardness Fracture and texture Grain size: Range and Dominant size Sorting Angularity Sphericity Matrix Cementation: Degree, Percentage of each cement and Composition. Accessories and Fossils: Type and Percentage of rock Effective Visual porosity Hydrocarbon indications Remarks: including Texture

This list contains those parameters which are considered relevant to a sample description. Not all parameters necessarily will be utilised in a description as the rock type governs this. However, all descriptions should be prepared by using the relevant parameters, in the order described. The ‘Cuttings Description Report’ is the primary recording tool for sample descriptions. The Wellsite Geologist in describing cuttings samples, will use the following aids in addition to the routine equipment and reagents available at wellsite. • •

GSA Rock Colour Chart Grain size Comparator

The sample descriptions will be entered into the ‘Cuttings Description Report’. The order in which these are entered is given above.

3.2

DISCUSSION OF ELEMENTS COMPRISING A LITHOLOGICAL DESCRIPTION 12

3.2.1

Sample Quality

It is inevitable that cuttings samples will be contaminated to some extent by cavings. The use of sieves can reduce this problem. Very large cuttings which are obviously caved may be removed from samples. Any wiper trip or round trip causes an increase in cavings when drilling is resumed and also causes a mixing up of the cuttings present in the mud column. For this reason, all cuttings should be circulated out of the hole prior to trips made near zones of interest. When it is not possible to catch samples (such as in lost circulation zones, an empty bag should be included in the sequence of samples, and clearly labelled to show the interval of missing samples and the reason why they are missing. Such intervals should be noted on the transmittal forms. 3.2.2

Rock Type

3.2.2.1 Siliciclastic The Siliciclastic classification detailed below is to be used when describing siliciclastic rocks.

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SILICICLASTIC CLASSIFICATION ROCK TYPES

MODIFYING CONSTITUENTS

UNCONSOLIDATED SEDIMENTS

CONSOLIDATED SEDIMENTS

MINERALS (> 20% OR IF SIGNIFICANT)

Gravels

Breccia and Conglomerate

Quartz Carbonate Arkose

Sands

Sandstone

Silts

Siltstone

Clays

Glauconitic Haematitic Arkosic (> 30% Feldspar) Feldspathic (10-30% Feldspar) Micaceous Anhydritic Pyritic Carbonaceous Cherty As for sandstones

Micaceous Haematitic/Limonitic Glauconitic Pyritic Gypsiferous Carbonaceous Chloritic Quartzose (silt size grains) Feldspathic (silt size feldspar) Dolomitic (Dolomite Rhombs)

Claystone/ Shale

14

3.2.2.2 Transitional The use of the term "grading" or “transitional” is informal. It is intended to describe the transition between fine-grained siliciclastic and carbonate rocks, as given below, or within one rock group, eg silty sandstone grading to sandstone.

Rock type

% Calcareous

% Clay

Calcilutite

80 – 100

0 - 20

Argillaceous Calcilutite

50 – 80

20 - 50

Calcareous Claystone

20 – 50

50 - 80

Claystone

0 - 20

80 - 100

The term Marl, a general “sack” term covering part or all of the range calcareous claystone to argillaceous calcilutite, is not one that OMV Australia chooses to use. 3.2.2.3 Carbonates The Carbonate Classification adopted in these procedures is that of Shields (1964). Wentworth Scale Grain Size

Lithological Name

Greater than sand sized Sand sized Silt sized Clay sized

Calcirudite Calcarenite Calcisiltite Calcilutite

3.2.2.4 Evaporites Evaporites are described according to the dominant evaporitic constituent, e.g. anhydrite, gypsum, halite, and dolomite. Lithological terms such as ‘dolostone’ are not used.

3.2.3

Colour

Colours should be those seen on wet cuttings and should be related to the GSA Rock Colour Chart. The rock sample and the Colour Chart need to be viewed under the same light source for consistency. It is important however, to ensure that only a significant colour differentiation is made in describing samples. It is common to make the colour description too elaborate and, effectively, meaningless. 15

3.2.4

Hardness

Loose Friable Soft Plastic Firm Moderately hard Hard

-

Very Hard

-

Dense

-

Brittle

-

3.2.5

Particles are discrete and non-coherent. Coherent, but crumbling under slight pressure. Clays, marls and silts which can be deformed by slight pressure. Pliant clays that show putty-like deformation. Compact, breaks under slight pressure. Grains can be detached using knife. Solidly cemented or lithified. Does not break under slight pressure, but can be scratched with knife blade. Fractures go between grains. Cannot be scratched with a knife blade, usually siliceous. Fractures pass through grains. Commonly used to indicate a fine-grained, well lithified tight rock (usually limestone) with sub-conchoidal fracture. Moderately hard, but breaks easily with firm pressure. Generally applies to shale with platy fracture.

Fracture

Several descriptive terms are used to describe the type of fracture, commonly a result of cleavage or bedding, seen in shale and limestone cuttings. They include: Blocky

Used to describe claystone, and limestone in which fractures are developed at approximately right angles, so that small blocks are formed.

Conchoidal

Commonly seen in dense rocks such as chert, argillite and flint. The term refers to the concave and convex surfaces developed on fractures. The fracture of hard limestone produces somewhat less strongly developed curved surfaces and the fracture has been called "subconchoidal".

Flaky

The rock fractures into small flakes or chips. Common in some argillaceous limestones and occasionally in metamorphic rocks.

Platy

Used to describe shale in which fissility is well developed. The rock breaks in parallel sided thin plates. This is commonly caused by fracture along bedding planes, or along cleavage directions.

Splintery

Used to describe shales in which the fissility is not strongly developed, but exists sufficiently to cause irregular surfaces and edges, like a board broken across the grain.

Be careful that apparent fracturing is not an artefact of the sample recovery process. 3.2.6

Texture 16

Texture is defined by the size, shape and arrangement of the component particles of a rock and much of the texture of a rock will have been described under the previous headings of grain size, shape and sorting. Other textural descriptions in general usage are: Rock Texture

amorphous, aphanitic, crystalline, dense, homogeneous, sucrosic, and vesicular.

Surface Texture of grains

Smooth: Rough:

flaky,

heterogeneous,

dull, nacreous, resinous, polished, and vitreous; etched, frosted, pitted, and striated.

In addition to the rock textures given above it is also useful to note here if the claystones react with water. The responses may be described as follows: Hygroturgid Hygroclastic Hygrofissile

3.2.7

swelling in a random manner dispersing as irregular fragments separating into tabular flakes

Grain size

The grain size comparison charts are related to the Wentworth scale. The grain size properties of a rock are defined by the range and dominant size of the constituents. Note that modifying constituents are also listed on this table.

Wentworth Scale Grade limits (diameters in mm)

Grain Size

Lithological Name

Above 256 256 - 64 64 - 4

Boulder Cobble Pebble

Conglomerate

4-2 2-1 1 - 1/2 1/2 - 1/4 1/4 - 1/8 1/8 - 1/16

Granule Very Coarse Coarse Medium Fine Very Fine

Sandstone

1/16 - 1/256

Silt

Siltstone

Less than 1/256

Clay

Claystone/Shale

Matrix will be described by type (silt, clay etc.) and proportion (%) of overall rock. See definitions of cement and matrix in Section 3.2.11. 3.2.8

Rounding

Standard charts (“comparators”) should be available at wellsite to assist in describing these properties. See Appendix 2 Roundness and Sphericity for the visual appearance of the descriptions below. The following definitions apply: Angular: Very little or no evidence or wear; edges and corners are sharp. corners, which are the minor convexities grain profile are numerous and sharp. 17

Secondary

Subangular: Definite signs of wear; edges and corners have been rounded off to some extent. Secondary corners are numerous. Subrounded: Showing considerable wear, edges and corners have been rounded off to smooth curves. The original shape of the grain is still distinct. Secondary corners are much reduced and rounded. Rounded: curves.

No original faces, edges or corners remain; the entire surface consists of broad

It is important that the description given should be of the original detrital grain. If the grain is affected by authigenic overgrowths, this should be noted and the concepts of angularity abandoned. 3.2.9

Sorting

The following classification should be used: Adjective

Definition

Very well Well Moderate Poor

90% 90% 90% 90%

of of of of

grains grains grains grains

in one grain size class, in two or three grain size classes, in four grain size classes, in five or more grain size classes.

As can be seen, a sandstone consisting entirely of very fine to fine grains cannot be poorly sorted. 3.2.10 Sphericity Sphericity should be considered when describing grain shape. Standard charts (“comparators”) should be available at wellsite to assist in describing these properties. The endpoints for description are Elongate and Spherical. See Appendix Sphericity for the visual appearance of these endpoints.

Roundness and

It is important that the description given should be of the original detrital grain. If the grain is affected by authigenic overgrowths, this should be noted and the concept of sphericity abandoned.

3.2.11 Cement Identified by type and effectiveness of the cement (calcite, quartz, dolomite etc.). Adjective

% of Pore Space Filled

Well Moderately Poorly

70-100% 30-70% 0-30% 18

Discussion (from AAPG Sample Examination Manual) Cement is a chemical precipitate deposited around the grains and in the interstices of sediment as aggregates of crystals or as growths on grains of the same composition. Matrix consists of small individual grains that fill interstices between the larger grains. Cement is deposited chemically and matrix mechanically. The order of precipitation of cement depends on the type of solution, number of ions in solution and the general geochemical environment. Several different cements, or generations of cement, may occur in a given rock, separately or overgrown on or replacing one another. The most common cementing materials are silica and calcite. Silica cement is common in nearly all quartz sandstones. This cement generally occurs as secondary crystal overgrowth deposits in optical continuity with detrital quartz grains. Opal, chalcedony and chert are other forms of siliceous cement. Dolomite and calcite are deposited as crystals in the interstices and as aggregates in the voids. Dolomite and calcite may be indigenous to the sandstone (the sands having been a mixture of quartz and dolomite or calcite grains) or the carbonate may have been precipitated as a coating around the sand grains before they were lithified. Calcite in the form of clear spar may be present as vug or other void filling in carbonate rocks. Anhydrite and gypsum cements are more commonly associated with dolomite and silica than with calcite. Additional cementing materials, usually of minor importance, include pyrite (generally as small crystals) siderite, haematite, limonite, zeolites and phosphatic material. Silt acts as a matrix, hastening cementation by filling interstices, thus decreasing the size of interstitial spaces. Clay is a common matrix material, which may cause loss of porosity either by compaction, or by swelling when water is introduced into the formation. Argillaceous material can be evenly distributed in siliciclastic or carbonate rocks, or have laminated, lenticular, detrital or nodular form. Compaction and the presence of varying amounts of secondary quartz, secondary carbonate, and interstitial clay are the main factors affecting pore space in siliciclastic rocks. While there is a general reduction of porosity with depth due to secondary cementation and compaction, ranges of porosity vary considerably due, primarily, to extreme variations in amounts of secondary cement. For instance, coarse-grained sandstones have greater permeability than finer ones when the same amount of cementing material is available to both. However, the same thickness of cement will form around the grains regardless of their size, therefore the smaller interstices, which occur in finer grained sandstones, will be cemented earliest. 3.2.12 Fossils and Accessories Reporting All fossils and accessories should be reported by type and relative abundance. The following adjectives can be used but actual percentages are preferred: Adjective Trace Rare Minor

Definition less than 1% 1 - 5% 5 – 10% 19

Common Abundant

10 - 20% 20% or greater

If the proportion of an accessory is greater than 20%, it is regarded as a modifier (See section 3.2.2). Discussion (from AAPG Sample Examination Manual) Microfossils and some small macrofossils, or even fragments of fossils, are used for correlation and may also be environment indicators. For aid in correlation, the Wellsite Geologist should record their presence and relative abundance in the samples being examined. More detailed identification will probably have to be made with the aid of the literature, and/or the advice and assistance of a palaeontologist. Fossils may aid the sample examiner in judging what part of the cuttings is in place and what part is caved. It would be helpful to the Wellsite Geologist to have available one or more slides or photographs illustrating the principal microfossils which might be expected to occur in each formation they will be logging. Accessory constituents, although constituting only a minor percentage of the bulk of a rock, may be significant indicators of environment of deposition, as well as clues to correlation. The most common accessories are glauconite, pyrite, feldspar, mica, siderite, carbonised plant remains, heavy minerals, chert, and sand-sized rock (lithic) fragments. 3.2.13 Visual Effective Porosity Visual porosity is the estimate of free pore space seen in drill cuttings under the microscope. It is a difficult, but important, parameter to evaluate. Generally, one cannot see the pore spaces under the binocular microscope, except in cases of very high porosity, and the observer must rely on other features for the porosity estimate. In general, if you can see the porosity it is very good to excellent. Unconsolidated sands are assumed to have very good porosity. However, beware of tightly cemented sands that have been fragmented by the bit and exhibit apparently good porosity. If you cannot see pores, there is a high percentage of matrix, the cuttings are smooth textured and the interval drilled relatively slowly, then the rock is likely to have poor porosity. The fair to good grades of porosity lie between these two described cases and experience will guide the observer. A useful technique is to describe cuttings of an offset well and to "calibrate" the descriptions of porosity with the wireline log data, prior to arriving at wellsite. Porosity does not systematically vary with the size of the particles making up the rock. Rocks with a fine grain size may be more porous than those with coarse grain size since porosity is defined as the percentage of pore space to the total volume of the rock. Factors such as sorting, packing/compaction, cementation and other effects determines ultimate effective porosity. Only the porosity of potential reservoir sequences should be described, as effective porosity is of interest. The porosity of claystone is irrelevant unless fractured. Approximate visual porosity grades should be denoted as: Excellent Good Fair Poor

20% and greater 15 - 20% 10 - 15% 5 - 10% 20

Nil (Tight)

0 - 5%

Porosity type can be described as: Intergranular - intercrystalline Vuggy - cavernous Fracture Solution. 3.2.14 Hydrocarbon Indications See Section 4. Show Evaluation

21

4 SHOW EVALUATION 4.1

INTRODUCTION:

Although petrophysical analyses may give a conclusive determination of the presence of commercial quantities of oil, it is the Wellsite Geologist's responsibility to report and log all shows and to ensure that shows are well evaluated. Positive indications of hydrocarbons in cuttings can be a decisive factor in the petrophysicist's evaluation of a well. Unfortunately, no specific criteria can be established as positive indications of whether or not a show represents a potentially productive interval. The colour and intensity of stain, fluorescence, cut, cut fluorescence and residual cut fluorescence will vary with the specific chemical, physical, and biologic properties of each hydrocarbon accumulation. The physical degradation of the shows (highly volatile fractions dissipate quickly), and flushing by drilling fluids or during sample washing, also tend to mask or eliminate evidence of hydrocarbons. The presence or absence of obvious shows cannot always be taken as conclusive. In many cases, the only suggestion of the presence of hydrocarbons may be a positive cut fluorescence. In other cases, only one or two of the other analytical methods may prove positive. Hence, when the presence of hydrocarbons is suspected, it is very important that all aspects be considered. For this reason a variety of detection methods have been described, together with recommendations for the ranking of the significance of shows. The following procedures have been directed, primarily, to the accurate description of shows in cuttings. It is axiomatic, however, that shows in cuttings are associated with shows in the mud systems. The levels of mud gas shows, their duration and composition, should accompany all descriptions of hydrocarbon shows. The rating of a hydrocarbon show (Sections 4.3 and 9.6) should attempt to reconcile the shows in the cuttings and in the mud. Fluorescence shows generally are severely limited in oil-based and synthetic oil-based muds (OBM and SBM). These muds mask genuine shows with background fluorescence associated with the oil/synthetic oil portion of the mud. Care and judgement must be exercised in these situations as the fluorescence seen is invariably from the mud or filtrate (see Appendix 5.6.2). 4.2

REPORTING OF SHOWS:

Hydrocarbon shows should be reported in the following format. 1.

Oil Stain (Section 4.6.1) Colour Amount minor (10%), moderate (10-50%), major (50-90%), saturated (100%) Distribution even, spotty, mottled, streaked etc.

2.

Odour (Section 4.6.2) Type Strength faint, moderate, strong (to be used cautiously - routine sniffing of samples poses a health hazard).

3.

Sample Fluorescence (Section 4.6.3) Colour Degree or intensity weak, moderate, bright Amount of sample (%) 22

Distribution

see "stain"

4.

Hydrocarbon (Solvent) Cut and Residue (Section 4.6.4) Cut Colour Residue Colour

5.

Solvent Cut Fluorescence (Section 4.6.5) Colour Degree or intensity Type of reaction (streaming [slow, moderate, fast] instant, blooming, crush etc.)

6.

Residual Cut Fluorescence (Section 4.6.5) Colour Degree or intensity

7.

Evaluation (Section 4.3) Trace, poor, fair, good, very good; possible/probable type of hydrocarbon

4.3

SHOW EVALUATION FORM

To assist the determination of show evaluation a form is included in Section 9.6. However, rather than being required as a formal reporting form, it is given as a check list for the geologist to use before advising the Operations Geologist and discuss its significance. It will also help to keep consistency during the discussions. The shows are to be reported on the Daily Geological Report with additional discussion in the “Comments” section if appropriate. 4.4

MUD GAS SHOWS

Mud gas shows will be recorded on the Total Gas Detector (Section 6.3) and the Gas Chromatograph (Section 6.4). Frequently, mud gas shows will appear slightly in advance of the cuttings with which they are related. If the mud rheology is optimal, however, there should be little delay between these two occurrences. 4.4.1

Total Gas Readings

Total gas readings are a measure of the amount of hydrocarbons entrained in the mud stream in the C1-C5 range. In common with the gas chromatograph, the upper limit of the detection system is constrained by the fact that C6 (hexane) and higher molecular weight hydrocarbons are liquid at surface conditions and are thus not carried in the air-stream from the gas trap to the Logging Unit. Total gas readings should be evaluated by the magnitude of deviations from a background value. The background gas, however, may change with mud chemistry and such sources should be monitored. In addition, there are several false hydrocarbon shows described in Section 6.10 about which the Wellsite Geologist should be familiar. Rapid deviations away from background gas levels represent a show. The gas chromatograph should be set to give good definition of the constituent hydrocarbons. An increase in trip gas or connection gas may signify that: 23

•

the mud weight has lowered and the well is being swabbed in by the drill string or

•

a new hydrocarbon zone has been penetrated and a review of other data sources (primarily cuttings) should be investigated.

4.4.2

Gas Chromatograph

The gas chromatograph (Section 6.4) analyses the proportion of each of the hydrocarbon gases (up to C5) measured by the Total Gas detector. The chromatograph readings allow for a semi-quantitative evaluation of hydrocarbon type by using several ratios calculated from the proportion of C1-C5. Each of the Mudlogging Contractors have their own proprietary methods for doing this. A useful reference however is Howarth et al (1984) from which the following indices are gleaned. Gas Wetness Ratio (GWR)% =

C2 + C3 + C4 + C5 x 100 -----------------------------C1 + C2 + C3 + C4 + C5

Light to Heavy Ratio (LHR)

C1 + C2 --------------C3 + C4 + C5

=

Oil Character Qualifier (OQR) =

C4 + C5 ---------C3

These ratios should be plotted by the Mud Logging Company for all intervals where gases heavier than C3 are recorded. The results are more meaningful if C5 is present. The numerical results of these ratios can assist in identifying the type of reservoired hydrocarbons (Appendix 1 Interpretation of Mud Gas Ratio Data). The Mudlogging Contractor will have the ability to compute these ratios directly from the input data. This makes life a great deal easier, particularly if GWR and LHR can be plotted on the same track on a log scale. Their manual will probably have a section describing more fully the significance of the ratios. 4.4.3

Integration with Shows from Cuttings

It is important that a hydrocarbon show be evaluated by reference to both the mud gas and cuttings data. The latter is the subject of Sections 4.6 and 4.7. Usually the two data sets are compatible and interpretation is straight forward. However, when the data sets appear to be in conflict: Check the mud gas detectors are working and are properly calibrated. Check the “Cuttings Description Report” for any anomalous data, and if the data set remains in conflict, THINK ...is the data is telling you something? In cases where shows are not good in cuttings (no stain, no cut and poor cut fluorescence) but a strong mud gas show is registered, a condensate may be responsible.

24

In cases where shows in cuttings are good (good even stain, strong cut and cut fluorescence) but no appreciable mud gas is detected, a very low GOR oil or biodegraded oil may have been encountered. It would be impractical to review all of the options available in this document, but a methodical review of the data set should yield a consistent result. 4.5

ROUTINE HYDROCARBON DETECTION METHODS FOR CUTTINGS

Much of the following text has been abstracted from Section 6 of the AAPG Sample Examination Manual. Minor changes have been made to make the qualitative description of shows match those utilised elsewhere in these procedures. 4.5.1

Staining and Bleeding

The amount by which cuttings and cores will be flushed on their way to the surface is largely a function of their permeability. In very permeable rocks only very small amounts of oil are retained in the cuttings. Often bleeding oil and gas may be observed in cores and sometimes in drill cuttings, from relatively tight formations. The amount of oil staining on ditch cuttings and cores is primarily a function of the distribution of the porosity and the oil distribution within the pores. The amount should be reported as a proportion of the sample as minor (