3D Seismic and Well Log Data Set Stratton Field

3-D SElsrviic AND

WELL LOG DATA SET Fluvial Reservoir Systems Stratton Field, South Texas Raymond A. Levey, Bob A. Hardage, Rick Edson, and Virginia Pendleton

Bureau of Economic Geology W. L. Fisher, Director The University of Texas at Austin Austin, Texas 78713-7508

sri

DOE

Gas Research Institute

1994

U.S. Department of Energy

LEGAL NOTICE This seismic and well log data set was prepared by the Bureau of Economic Geology as an account of work sponsored by the Gas Research Institute (GRI). Neither GRI, members of GRI, nor any person acting on behalf of either a.

Makes any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this seismic and well log data set, or that the use of any information, apparatus, method, or process disclosed in this seismic and well log data set may not infringe privately owned rights, or

b.

Assumes any liability with respect to the use of, or for any and all damages resulting from the use of, any information, apparatus, method, or process disclosed in this seismic and well log data set.

This technology transfer product contains the following: • Text describing the geologic framework, seismic acquisition parameters, data volumes, and installation instructions. • An 8-mm tape with a SEGY-formatted 3-D seismic data volume for 2 mie. • A 3.5-inch disk with 24 ASCII-formatted data files.

CONTENTS Purpose

1

Introduction

1

Geologic framework

3

Regional structural and stratigraphic setting of South Texas

3

Structural and stratigraphic setting of Stratton field Reservoir characterization of the 3-D seismic and

6

well log data area

9

Reserve growth in fluvial reservoirs of Stratton field Seismic acquisition parameters

10 11

Binning

11

Data processing

11

Data volumes

13

3-D seismic data set

13

Well log data set

14

Vertical seismic profile (VSP) calibration data

19 28

Installation instructions 3.5-inch disk

28

8-mm tape Acknowledgments

28 29

References

29

iii

FIGURES 1. Depositional dip-oriented cross section through the Texas Gulf Coast Basin illustrating the relative position of major sand depocenters 2. Chronostratigraphic and lithostratigraphic chart showing the relative rank of the volume of gas produced in the major Cenozoic depositional episodes of the onshore Gulf Coast Basin 3. Location map showing the outline of FR-4 gas play and position of Stratton—Agua Dulce field, Nueces and Kleberg Counties, Texas 4. Depositional framework of the Frio Formation 5. Subregional dip-oriented reflection seismic line through Stratton field showing the rotation of fault blocks in the lower Frio and Vicksburg Formations 6. Reservoir nomenclature of the Vicksburg and Frio Formations in the Stratton—Agua Dulce field complex, Nueces and Kleberg Counties, Texas 7. Map of receiver-array geometry used to create uniform ground coverage along each receiver line and side and map views of vibrator source array and pad positions 8. Location of well and seismic lines in the 2-mi2 study area 9. Calibration of reservoirs using the vertical seismic profile in well no. 9

3

4 5 6

7

8

12 13 15

TABLES 1. Well log types for the 21 wells in the data set 2. Depths and seismic two-way times of 10 middle Frio reservoir horizons in 21 wells 3. Time-versus-depth calibration determined from vertical seismic profile in well no. 9

iv

14 16 20

PURPOSE This seismic and well log data set is part of a technology transfer program conducted by the Bureau of Economic Geology on behalf of the Gas Research Institute and the U.S. Department of Energy. This product provides a versatile data set that can be used for testing hardware and software products, training individuals in the use of 3-D seismic data and well log information, and ultimately illustrating the complex stratigraphy and structure of natural gas reservoirs. This technology transfer product is intended to assist gas operators and contractors in developing the skills and tools to deploy technologies that will ensure an adequate supply of natural gas resources at competitive prices. This 3-D seismic and well log data set is from Stratton field in South Texas.

INTRODUCTION A diverse and vast resource base of 1,295 Tcf of technically recoverable natural gas resources (including proved reserves, conventional resources, and nonconventional resources) is estimated by a recent analysis of domestic petroleum supplies by the National Petroleum Council (1992). Of this resource base, 230 Tcf is predicted to be recoverable by reserve appreciation from conventional and tight gas reservoirs in the lower 48 states. The integrated application of concepts and cost-effective technologies from the disciplines of geology, engineeri ng, geophysics, and petrophysics will be required to convert these natural gas resources into producible reserves. In the last decade, characterization of the internal geometry of reservoirs, mainly oil reservoirs, has demonstrated a higher degree of compartmentalization than previously recognized. Nonstructural compartmentalization is primarily a function of the depositional system and, secondarily, of the diagenetic history of the reservoir. The research objective of the Bureau of Economic Geology infield reserve growth project is to define the potential for incremental gas recovery through better understanding of depositional and diagenetic heterogeneity within nonassociated natural gas reservoirs. Keywords: Agua Dulce field, data set, fluvial environment, seismic survey, Stratton field, three-dimensional seismic, vertical seismic profile (VSP), vibroseis, Vicksburg, well log

1

Addition of natural gas resources from reserve growth and reserve appreciation in conventional reservoirs have multiple components. Historically, industry has added infield reserves by extensions and deeper pool drilling. Recompletions of existing wells were often made without considering the concepts of reservoir heterogeneity or compartmentalization. Where significant geologic variation occurs, untapped, incompletely drained, or bypassed reservoir compartments remain to be drained of natural gas by the drilling of new strategically placed infield wells or by selective recompletions in existing wells. Future infield reserve growth will be based on an understanding of vertical and lateral heterogeneity that leads to recompletions in bypassed and incompletely drained reservoirs. In addition to concepts of reservoir heterogeneity and compartmentalization, new surface and downhole tools are being developed that will enhance the operator's ability to define resource targets with greater precision and reliability. Surface 3-D seismic is one of the readily available state-of-the-art technologies for imaging stratigraphically and structurally complex reservoirs. An increasing trend in the lower 48 states is the reexploration and redevelopment of large known fields with an existing infrastructure. The viability of this trend has important implications for our future domestic gas supply as both major companies and independents seek to recover the substantial remaining gas resources identified in the resource assessment completed by the National Petroleum Council (1992). Evaluation of natural gas resources is part of a continuing research initiative at the Bureau of Economic Geology focusing on strategies to maximize the potential for natural gas production in the lower 48 states. The Secondary Natural Gas Recovery: Targeted Technology Applications for Infield Reserve Growth research project is a joint venture sponsored by the Gas Research Institute, the U.S. Department of Energy, and the State of Texas through the Bureau of Economic Geology at The University of Texas at Austin, with co-funding and cooperation of the natural gas industry. This project is a field-based program using a multidisciplinary approach that integrates geology, geophysics, engineering, and petrophysics. A major objective of this project is to develop, test, and verify those technologies and methodologies that have near- to mid-term potential for maximizing gas recovery from conventional reservoirs in known fields. Siliciclastic natural gas reservoirs, which yield 50 percent of the cumulative natural gas production in the Gulf Coast Basin of Texas, are targeted as data-rich field-based models for evaluating infield development.

2

▪

GEOLOGIC FRAMEWORK REGIONAL STRUCTURAL AND STRATIGRAPHIC SETTING OF SOUTH TEXAS The Oligocene Frio Formation is one of the major progradational offlapping stratigraphic units in the northwest Gulf Coast Basin (fig. 1). The Frio Formation is a sediment-supply-dominated depositional sequence characterized by rapid deposition and high subsidence rates (Galloway and others, 1982; Morton and Galloway, 1991). The thickness of the Frio Formation ranges from less than 2,000 ft near the Vicksburg Fault Zone to greater than 9,000 ft downdip toward the central part of the Rio Grande Embayment. The Rio Grande Embayment was one of several entry points for major river systems that drained into the ancestral Gulf of Mexico. The structural style of the Rio Grande Embayment is characterized by discontinuous belts of strike-parallel growth faults, deep shale ridges and massifs, and underlying salt that is thin or absent (Galloway and others, 1982). The Oligocene Frio Formation of Texas is volumetrically the largest gas-productive interval of 10 major depositional stratigraphic packages in the Cenozoic Gulf Coast Basin (fig. 2). Northwest

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The middle Oligocene section was deposited during the Catahoula-Frio depositional episode described by Galloway (1977). The entire Frio Formation is divided into 10 gas plays on the basis of regional variations in structure and depositional setting (Kosters and others, 1989). The Frio Fluvial/Deltaic Sandstone play along the Vicksburg Fault Zone (gas play FR-4 from Kosters and others, 1989) is ranked as the third largest of all 73 gas plays in Texas (fig. 3). Gas play FR-4 is the largest of all nonassociated gas plays in the Gulf Coast; cumulative production had exceeded 12 Tcf as of January 1, 1991. Hydrocarbon trapping in reservoirs of the FR-4 gas play is controlled by a combination of structural and stratigraphic factors, including faulted anticlinal closure, facies change, and reservoir pinch-out (Kosters and others, 1989). Most of the gas production in the FR-4 play is from middle Frio reservoirs. These reservoirs in Stratton field are part of the Gueydan fluvial system (fig. 4), updip from the Norias delta system (Galloway and others, 1982).

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STRUCTURAL AND STRATIGRAPHIC SETTING OF STRATTON FIELD The 3-D seismic and well log data set is from Stratton field, which lies near the northern limit of the FR-4 gas play (fig. 4). Variation in the structural framework of Frio and Vicksburg reservoirs in Stratton field is illustrated by a structural dip-oriented seismic line crossing the Vicksburg Fault Zone and extending across the study area (fig. 5). Structural attitude of the gas reservoirs in the Vicksburg and lower Frio Formations is affected by a series of normal faults that sole out into the Vicksburg detachment zone within the Jackson Shale. The relatively undeformed and flat-lying middle and upper Frio Formation is structurally much simpler than the underlying Vicksburg and lower Frio, which show the effects of structural rotation above the Vicksburg décollement surface, including antithetic faulting. The middle and upper Frio are characterized by a relatively gentle north-to-south-elongated subsurface domal closure. Analysis of both well log correlations and 2-D reflection seismic data indicates that only a few faults extend upward into the middle Frio and that the amount of fault throw in the middle Frio is usually small and fault block rotation minimal. A large part of the middle and upper Frio is relatively undeformed across Stratton field (fig. 5). A type log for Stratton field illustrates the distinct stratigraphic variation between the Vicksburg and Frio Formations (fig. 6). 6

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Figure 5. Subregional dip-oriented reflection seismic line through Stratton field showing the rotation of fault blocks in the lower Frio and Vicksburg Formations. This faulting contrasts sharply with the lack of faulting in the middle and upper Frio Formation.

Analysis of conventional electric wireline logs (spontaneous potential [SP] and resistivity) indicates that the middle Frio contains multiple stacked pay sandstones within a series of vertically stacked reservoir sequences referred to as the B-, C-, D-, E-, and F-series reservoirs (descending stratigraphic order). Reservoir facies of the middle Frio examined in core and borehole images and calibrated to well logs are interpreted as multiple amalgamated fluvial channel-fill and splay sandstones. Channel-fill deposits range from 10 to 30 ft in thickness and show either an upward-fining or a blocky SP log profile. Lateral splay deposits commonly range from less than 5 ft to as much as 20 ft in thickness. The SP log of splay deposits has a classic upward-coarsening textural profile. The lower Frio Formation in the south part of Stratton field occurs at depths between approximately 7,000 and 7,400 ft subsea. These 7

SP Res Match line

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Match line

Figure 6. Reservoir nomenclature of the Vicksburg and Frio Formations in the StrattonAgua Dulce field complex, Nueces and Kleberg Counties, Texas (modified from Kling, 1972). Ten middle Frio reservoir horizons in table 2 are shown here in larger type; other reservoirs are shown in smaller type.

sandstones range from 5 to 15 ft in thickness and show an upward-coarsening

profile on the SP log. These lower Frio Formation sandstone packages are interpreted as lower coastal plain to inner shelf deposits (Kerr, 1990) and are probably related to the Norias wave-dominated delta system delineated by Galloway and others (1982). Correlation of fieldwide resistivity and conductivity markers across Stratton field indicates that these sandstone packages are equivalent to the G-series reservoirs (fig. 6). 8

The Vicksburg Formation contains siliciclastic intervals with SP and gamma-ray logs indicative of upward-coarsening grain-size profiles consistent with deltaic-dominated depositional systems. Genetic stratigraphic analysis by Han (1981), Han and Scott (1981), and Langford and others (in press) identified vari ations in delta morphology within the middle and upper Vicksburg Formation that are thought to represent mostly dip-oriented fluvial-dominated deltas. These sandstones in the upper Vicksburg Formation are commonly greater than 20 ft in thickness (up to 80 ft thick) and show a distinctive upwardcoarsening SP log profile, representative of progradational deltaic depositional environments.

RESERVOIR CHARACTERIZATION OF THE 3-D SEISMIC AND WELL LOG DATA AREA This seismic and well log data set focuses on natural gas reservoirs in the middle Frio Formation. Reservoir facies of the middle Frio fluvial strata are channel-fill and associated crevasse-splay sandstones. Criteria for identifying these channel-fill and splay deposits from well logs and cores were described by Jirik (1990), Kerr (1990), and Kerr and Jirik (1990). Several giant gas fields (cumulative production more than 1.5 Tcf) that are part of this play include Seeligson and the Stratton—Agua Dulce field complex. Geologic analysis of middle Frio reservoirs in these fields shows that composite channel-fill deposits are as much as 30 ft thick and 2,500 ft wide and probably occur in long linear belts (Kerr and Jirik, 1990). Splay deposits in the middle Frio are up to 20 ft thick and are proximal to channel systems (Kerr, 1990). Porosities in these fluvial reservoirs range from 15 to 25 percent, with air permeabilities of less than 1 to greater than 4,000 md. Analysis of 3-D seismic imagery and multiple well tests were used to investigate reservoir compartment boundaries identified by geologic, engineering, and formation evaluation techniques. Data volume flattening and seiscrop slicing above and below reference hori zons were used to reveal depositional topography and to determine the extent of structural influence on reservoir horizons. Seiscrop imagery and well log analysis have revealed fluvial channels as narrow as 200 ft and as thin as 10 ft at depths as great as 6,750 ft. Some reservoir compartment boundaries were seismically imaged within individual channel systems. Because many of these gas reservoirs are only 10 to 15 ft thick and occur within seismic time windows as thin as 2 to 4 milliseconds (ms), careful calibration of subsurface stratigraphy with the surface

9

seismic response is required to accurately locate these narrow time windows in the reflection waveforms. This research has demonstrated that a properly calibrated 3-D seismic data volume provides a cost-effective tool for imaging fluvial reservoir topology in the interwell space. Using pressure buildup test data in middle Frio fluvial reservoirs, project-designed well tests identified long and narrow drainage shapes. These channellike drainage shapes have an average width of 220 ft and range from a minimum of 50 ft to a maximum of 600 ft, matching the dimensions of narrow thin-bed reservoirs imaged on seiscrop slice maps. These well tests confirm geologically based reservoir characterization models and indicate that significant incremental gas resources are trapped by reservoir compartments in fluvial-channel systems (Levey and others, 1992).

RESERVE GROWTH IN FLUVIAL RESERVOIRS OF STRATTON FIELD Analysis of reserve growth indicates more than 90 percent reserve replacement after 40 years of production and development across a large contiguous lease block in the south part of Stratton field (Sippel and Levey, 1991; Levey and others, 1993). Deeper pool drilling that was targeted for deltaic reservoirs in the lower Frio and Vicksburg Formations led to recognition of the potential for higher density development of shallower intervals in the middle Frio. Historical analysis of reserve additions indicated that the fluvial reservoirs less than 7,000 ft deep resulted in most of the reserve appreciation. This reserve growth was achieved from the shallower, middle Frio reservoirs and is superior to that obtained by completions in the deeper reservoirs in the lower Frio and Vicksburg, both on a total gas volume basis and on a percompletion comparison. The contrast in gas production between reserves shallower than 7,000 ft and those deeper than 7,000 ft coincides with a major change in the structural and depositional setting in the study area. The middle Frio Formation reservoirs (7,000 ft) are in a predominantly deltaic setting that penetrates the top of geopressure. In addition, structurally rotated fault blocks are encountered in the lower Frio and Vicksburg below 7,000 ft. It is well recognized that drilling costs almost always increase when (1) drilling deeper wells, (2) penetrating geopressured intervals, and (3) drilling in structurally deformed intervals. Estimates of incremental recoverable gas range from less than 0.5 to 2.0 Bcf per reservoir completion in the middle Frio.

10

SEISMIC ACQUISITION PARAMETERS The 3-D source/receiver geometry consisted of east-west receiver lines spaced 1,320 ft apart and north-south source lines spaced 880 ft apart. Linear, 12-element receiver arrays were constructed so each array spanned 110 ft. The distance between the centers of adjacent arrays was 110 ft, which means there was a continuous ground coverage of uniformly spaced geophones along each receiver line (fig. 7). Vibrator points (VP's) were spaced at intervals of 220 ft. Six sweeps from four inline vibrators were summed at each VP. A linear, 8-120 Hz sweep rate was used, and the sweep length was 14 s. The four vibrators were stationed bumper to bumper with their pads separated 35 ft to create a source array 135 ft long. To minimize ground damage, the vibrators moved forward one pad width after each sweep. At each VP, the final positions of the 24 pad imprints created by the four-element, six-sweep summation were symmetrically distributed on each side of the source flag (fig. 7).

BINNING The receiver spacing of 110 ft and the source spacing of 220 ft created an acquisition bin measuring 55 by 110 ft. After the 3-D data volume was stacked, the data were interpolated in the source line direction to create interpretation bins measuring 55 by 55 ft.

DATA PROCESSING The major data processing steps were zero-phase correction of field records, prestack Q compensation to emphasize higher frequencies, surface consistent deconvolution and statics, iterative velocity analysis, stacking, and one-pass 3-D wave equation migration.

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12

DATA VOLUMES All of the data volumes presented are from a 2-mie area in Stratton field. The 3-D seismic grid and well locations are shown in figure 8. 100

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3-D SEISMIC DATA SET The migrated 3-D data volume is written in SEGY format. It comprises 100 inlines (oriented east-west) and 200 crosslines (oriented north-south). The crossline length is 3 s, and the sample rate is 2 ms. Crossline 1 of inline 1 lies in the southwest corner of the grid; crossline 200 of inline 100 defines the northeast corner of the grid. Trace spacing is 55 ft both from east to west and from north to south. The minimum (inline, crossline) _ (1, 1) position should be set at (x, y) coordinates (0, 0), and the maximum (inline, crossline) _ (100, 200) position should be at coordinates (x, y) _ (10945, 5445). The inline number is stored in bytes 9-12 of the trace header, and the crossline number is in bytes 13-16. The amplitude data are stored in 32-bit IBM floating-point format, and the maximum amplitude is 5715.

13

Table 1. Well log types for the 21 wells in the data set with their abbreviations and units of measure.

Units

Abbreviation

Log name

ASN CALIPER DEPTH DRHO

amplified short normal caliper depth delta rho correction on density log gamma ray guard induction log—deep induction log—medium lateral long normal microinverse micronormal neutron porosity (sandstone matrix) bulk density true resistivity spherically focused log short guard short normal spontaneous potential

GR GRD

LLD ILM LAT IN MINV MNOR NPSS RHOB RT SFLU SGRD SN SP

ohm meters inches feet grams per cubic centimeter American Petroleum Institute ohm meters ohm meters ohm meters ohm meters ohm meters ohm meters ohm meters porosity units grams per cubic centimeter ohm meters ohm meters ohm meters millivolts ohm meters

WELL LOG DATA SET A total of 21 wells, including a vertical seismic profile, from the 2-mi2 area are provided in this data set on 3.5-inch disk. As is typical of many gas fields with a long production history, the log suites vary in the type of logging tools used and in the quality of the data acquired. A list of the abbreviations of various log types in the data set is shown in table 1. The depths of 10 different middle Frio reservoir horizons (fig. 9) are provided for each of the 21 wells (table 2). These 10 reservoirs produce natural gas in Stratton field.

14

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