Reservoir Mapping Research Articles

Strategies for Optimizing Incremental Recovery from Mature Reservoirs in Oligocene Frio Fluvial-Deltaic Sandstones, Rincon Field, South Texas: ABSTRACT

ABSTRACT Fluvial-deltaic sandstones represent a large percentage of reservoirs in U. S. fields that face premature abandonment, despite having an estimated 30 billion barrels of unrecovered oil. These heterogeneous sandstones possess excellent potential for incremental recovery of significant oil volumes currently isolated in untapped and incompletely drained reservoir compartments. Results from geological and petrophysical characterization of selected Frio reservoirs are being used both to identify specific infill and recompletion opportunities and to guide incremental recovery strategies in mature reservoirs throughout South Texas that are now on the verge of being abandoned. Reservoir mapping and stratigraphic facies analysis in Rincon Field were used to describe depositional styles present within a 150-ft sequence that has produced more than 24 million barrels of oil since 1940. The distribution and geometry of more than eight separate flow units in the Frio D-E reservoir interval record aggradational channel-fill sedimentation within an overall backstepping pattern, followed by development of strike-oriented shoreface bars, and a return to channel deposition in an overall progradational regime. Differences in sandstone architecture between retrogradational and progradational units result in different degrees of reservoir compartmentalization that directly impact recovery efficiency. In the retrogradational Frio E reservoir, sandstone thickness is greatest in an updip position relative to the crest of the structure, creating potential for increased flow communication between dominantly dip-oriented channel units that results in a relatively high recovery efficiency of 38%. In contrast, Frio D reservoir units exhibit more complex facies patterns within a net progradational sequence of narrower, less interconnected channel-fill deposits, with areas of maximum sediment thickness located downdip from the structural crest, thus limiting opportunities for lateral flow communication that has resulted in a lower (29%) recovery efficiency. Petrophysical modeling based on evaluation of abundant core data and saturation estimates from special core analyses is being used to describe permeability structure, map remaining oil distribution, and target undeveloped reservoir compartments with large oil volumes. Better understanding of controls on the stratigraphic distribution of oil in these sandstones and its subsequent production has wide application to many other analogous reservoirs and is a key to optimizing incremental recovery in mature Frio reservoirs.

The Impact of Advances in Seismic Technology on Reservoir Mapping: ABSTRACT

The Impact of Advances in Seismic Technology on Reservoir Mapping: ABSTRACT

Applications of Computer Spreadsheet Programming for Interactive Pickett Plot Petrofacies Analysis and Mapping of Hydrocarbon Reservoirs: ABSTRACT

Applications of Computer Spreadsheet Programming for Interactive Pickett Plot Petrofacies Analysis and Mapping of Hydrocarbon Reservoirs: ABSTRACT

De Wijk Gas Field: Reservoir Mapping with Amplitude Anomalies

De Wijk field, discovered in 1949, is located in the northeastern part of Netherlands. The main gas accumulation is contained in cretaceous and Triassic sandstone reservoirs trapped in a broad salt-induced structure of around 80 km[sup 2] areal extent. The field contains gas in the tertiary, Chalk, Zechstein 2 Carbonate, and Carboniferous reservoirs as well. De Wijk field is unique in the Netherlands as most gas-producing reservoirs in the Cretaceous/Triassic are of no commercial interest. Post-depositional leaching has positively affected the reservoir properties of the Triassic formations subcropping below the Cretaceous unconformity. Optimum, interpretation of 3-D seismic data acquired in 1989 resulted in spectacular displays highlighting the uniqueness of the field. Most gas-bearing reservoirs are expressed on seismic by amplitude anomalies. Various attribute-measurement techniques show the effect of gas fill, leaching, and sand distribution in the various reservoirs.

USE OF HEAVY MINERAL SUITES IN RESERVOIR SAND STUDIES IN THE HARRIET FIELD, BARROW SUB-BASIN, WESTERN AUSTRALIA

Analysis of the heavy mineral suites in the reservoir at Harriet Field has significantly improved reservoir unit definition and correlation and provided information on facies changes and diagenetic history. It has provided further evidence for a stratigraphic barrier as a cause of the variation of the oil-water contact in the field.The reservoir consists of a number of discrete sandstone bodies which are arranged in a multistorey manner.The reservoir is further subdivided into compartments by minor faulting. Prior to the use of heavy mineral analysis, correlation between wells was often unclear. Such correlation is beyond the resolution of reflection seismology and the massive nature of the sandstones means that definition and correlation is uncertain when made on the basis of lithology combined with wireline logs. The time interval involved in the deposition of the reservoir sequence is too short to permit discrimination by palaeontological analysis.Eight distinct sandstone bodies can be defined on the basis of analysis of the heavy mineral suites in the 14 wells of the field. The total composition of the suites, certain 'marker minerals' and various statistical indices have been used to define these sandstone units which are interpreted to be individual lobes within a submarine fan complex. The methods and results are illustrated with examples from the field. The results of the analysis show that heavy mineral populations can provide critical information for accurate reservoir mapping and analysis.

New method for reservoir mapping : Journel, A G; Alabert, F G J Pet TechnolV42, N2, Feb 1990, P212–218

New method for reservoir mapping : Journel, A G; Alabert, F G J Pet TechnolV42, N2, Feb 1990, P212–218

New Method for Reservoir Mapping

Summary The sequential indicator simulation (SIS) algorithm allows buildingalternative, equiprobable, numerical models of reservoir heterogeneities thatreflect spatial connectivity patterns of extreme values (e.g., permeability)and honor data values at their locations. This paper presents a case study of asampled presents a case study of a sampled slab of Berea sandstone. Introduction A map or, more generally, a numerical model of an attribute's distributionin space is rarely an end goal. Maps are used as input to some transferfunction designed to simulate a response of interest (Fig. 1). The"quality" of a map or numerical model should be appreciated in relationto the particular transfer function through which it will particular transferfunction through which it will be processed. A map is a poor model of realityif it does not reflect those characteristics of the real spatial distributionthat most affect the response function. The goal of reservoir characterization is to provide a numerical model ofreservoir attributes (porosity, permeability, saturations, etc.) for input intocomplex transfer functions represented by the various flow simulators. Thereservoir model is "good" if it provides response functions similar tothose that would be provided by a perfect model based on an exhaustive samplingof the reservoir (see Fig. 1). Aspects of the reservoir that have littleinfluence on the response of the flow-simulation exercise considered need notbe modeled; however, reproduction of critical reservoir aspects isessential. In most flow simulations, the single most influential input is thepermeability (or transmissivity) spatial distribution that conditions flowpaths. In a heterogeneous reservoir involving layers (e.g., shales/sands/fractures) with permeability values differing by several orders of magnitude, flow is conditioned primarily by connected paths of high or low permeabilityvalues (flow paths and barriers, respectively). The histogram shape, whetherlog normal or not, and the proportions of extreme permeability valuesproportions of extreme permeability values do not matter as much as the spatialconnectivity of these extreme values. Randomly disconnected small fractures maynot generate flow paths, whereas a minute volume proportion of connected highpermeability proportion of connected high permeability values may control flowand thus sweep efficiency and recovery. In such situations, reservoircharacterization should detect patterns of connected extreme values andpatterns of connected extreme values and represent them in the numericalmodel(s) to be used for flow simulations. In the following we argue thattraditional mapping criteria, such as smoothness of the resulting surface orminimum-error variance, may not be relevant because they are not related toreservoir connectivity. Focusing on spatial connectivity of extreme-valued attributes involves ahigh degree of uncertainty that must be assessed. For example, if a given mapor numerical model features a string of high permeability values, is thatstring an artifact of the geological interpretation or the interpolationalgorithm used? Assessing spatial uncertainty is much more demanding thanassessing the local accuracy of all estimated values along the stringconsidered. Our solution does not provide a single estimated map, as in Fig.provide a single estimated map, as in Fig. 1b, but several alternative, yetequiprobable, maps, as in Fig. 1c. All such maps honor the data values at theirlocations and reproduce a certain number of connectivity functions that modelthe dependence in space of the attribute considered; map differences image theprevailing spatial uncertainty. Features that appear on all maps are deemedreliable, and those that appear only on some maps unreliable, although theirpossible occurrence elsewhere cannot be ruled out. The reservoir engineer thencan make a management decision with some awareness of the risk imparted byspatial uncertainty. Finally, "hard" data on extreme values are scarce or nonexistent andmust be supplemented by "soft" information. For example, because plugsare not taken in shaly or fractured parts of the core, core plugs fail tosample extreme values, thereby biasing the permeability distribution. Loginterpretation, however, can indicate the presence of such extreme values. Information comes from various sources at various scales with various degreesof reliability, yet all information sources must be accounted for when dealingwith the spatial distribution of extreme values. Indicator formalism allows numerical or interpretive information to becommonly coded into elementary bits (valued at zero or one). These bits arethen processed independently of their origins to generate the requirednumerical models of the reservoir.

Geophysical Evaluation of the Yacheng 13-1 Natural Gas Field, People's Republic of China: ABSTRACT

The Yacheng 13-1 natural gas field was discovered by ARCO China in 1983 with the drilling of the Yacheng 13-1-1 well. Since that time an additional 5 wells have been drilled to define the field. Current mapping of the field indicates there are 3,299 BCF gas-in-place in the main reservoir. Geophysics have been central in the process of assessing field reserves. It has been critical in the structural mapping of the field, as well as defining the stratigraphic character and elements of the reservoir. The 2-D seismic acquired in 1979, 1983, 1984, and 1985 provide a 1 km grid of data. This grid has been sufficient to characterize the field in a gross sense; however, only with the use of state-of-the-art processing methods has ARCO China been able to maximize the benefit of these data in assessing the reservoir distribution, gas/fluid contacts and well information. In 1989, ARCO reprocessed all of the seismic lines over the field using the Pre-Stack F/K Migration technique. This improved the structural image and the resolution of stratigraphic elements. From these improvements, a better understanding of the relationship between faulting and basin depositional history and hydrocarbon contacts within the 13-1 Sand member has resulted. Furthermore,more » their reprocessing has allowed detailed mapping of additional potential reservoirs within the overlying wedge member.« less

Detailed Mapping of Reservoir Structural Geometry in Detached, Shortened Fold Belts, Ortega (Aptian-Albian) Field Area, Girardot Basin, Colombia: ABSTRACT

Discovery and development of oil fields in shortened compressional fold belts require recognition that the largest reservoirs occur in intermediate or third-order scale folds. Third-order folds generally are preserved in the footwall of larger scale fold thrusts, and may be concealed beneath smaller, detached surface folds of nonreservoir condition. A successful reservoir mapping procedure involved (1) detailed surface mapping, (2) depth conversion of seismic data (3) construction of a network of true-scale balanced cross sections, and (4) contour mapping above and below the major zones of detachment. Structure at the 11 million bbl Ortega field consist of convergent third-order fold thrusts, with internal decollement. Tight flexural-slip folds imbricate and tectonically thicken upward on the west-verging Ortega anticline. The buried, east-vergent Salado anticline acts as a buttress to westward propagation at Ortega but retreats along strike to allow the Porvenir anticline to develop a low, broad, fault-bend fold geometry. At least four fault blocks in the Ortega field remain untested because balanced section analysis has not been employed to create additional control points for structure contour maps on the top of the reservoir. It is speculated that another 10-50 million bbl of primary recoverable reserves remain in the Ortega field. Inmore » addition, reservoirs like the Ortega field should occur elsewhere in the Girardot basin. These reservoirs likely will occur all along the footwall of fold-thrust structures on the flank of the intrabasin Pata high.« less

Examples of Reservoir Maps and Cross Sections Depicting Well-to-Well Continuity and Reservoir Heterogeneity Based on Three-Dimensional Computer Models: ABSTRACT

Three-dimensional computer models are extremely useful in providing a complex reservoir description for reservoir simulation. In addition, these models are a source of contour maps of porosity, permeability, and fluid distributions as well as lithological cross sections. Such displays allow the geologist to check the accuracy of an interpretation as represented in the computer model and enable the reservoir engineer to more efficiently select the optimum areas and procedures for reservoir simulation. This poster session shows examples of displays normally provided to assist in the reservoir simulations, sections and contour maps useful in assessing well-to-well continuity, and sections of models containing random heterogeneity.

Hydrocarbon Reservoir Potential of the Taranaki Basin, Western New Zealand

Taranaki Basin is a proven petroleum producing region, with commercial quantities of hydrocarbons from late Eocene paralic and terrestrial sands, and Miocene-latest Pliocene shelf sands. Other sediments with sub-commercial hydrocarbon accumulations, shows or potential reservoir features have also been encountered. The paralic and terrestrial sediments were deposited during periodic shoreline fluctuations in the Paleogene and were capped by transgressive terrigenous and carbonate muds. Other sand bodies, generally of bathyal and shelf setting and representing increasing regional tectonism, are found throughout the late Eocene to Pliocene sequence. Paleogeographic reconstructions depicting the maximum sand development during the Paelocene to Pliocene provide potential sandstone reservoir maps. These highlight onshore Taranaki and the Eocene paleoshoreline trend as areas of greatest prospectivity. Future activity should also consider the potential of the relatively unexplored late Cretaceous-Paleocene and Pliocene sandstone sequences.

Crosswell Logging for Acoustic Impedance

Crosswell seismic data are recorded by placing the seismic source in one well and receivers in a second well to measure physical properties between the two wells. When reflected seismic energy is extracted from the recorded seismograms, the resultant picture is a function of acoustic impedance (density times velocity) between the wells. Previous crosswell seismic studies have predominantly used direct waves and measure only seismic velocities by tomographic techniques. Synthetic crosswell seismic data are generated by a computer to illustrate the concepts and procedures used for crosswell impedance logging in this study. Such synthetics are useful for investigating how the experiment should work, but real data reveal how the experiment does work. Crosswell seismic data recorded at the Multiwell Experiment (MWX) site in western Colorado revealed a few surprises when compared with synthetic data. Real data consist of a lot more than simple direct arrivals and reflections. There are numerous converted waves, borehole (tube) waves, and general noise. Source directivity proves to be a very limiting problem, because energy is concentrated in a horizontal plane perpendicular to the borehole. The final product of crosswell logging for acoustic impedance is a seismogram similar to the classic CDP seismogram recorded on themore » surface for exploration. The crosswell data in this paper are in the kilohertz range as opposed to surface data, which are generally below 100 cycles/sec (100 Hz). Seismic data in the kilohertz range can theoretically resolve layers as thin as 2 ft (0.6 m) vertically. Actual resolution indicated is about 5 to 10 ft (1.5 to 3 m).« less

Seismic computations on the IBM 3090 Vector Multiprocessor

Computerized seismic prospecting is an echo-ranging technique usually targeted at accurate mapping of oil and gas reservoirs. In seismic surveys an impulsive source, often an explosive charge, located at the earths surface generates elastic waves which propagate in the subsurface; these waves are scattered by the earths geological discontinuities back to the surface, where an array of receivers registers the reflected signals. The data recorded are then processed in a complex sequence of steps. Among them, seismic migration and stacking velocity estimation represent two characteristic components of the process solving the inverse problem of recovering the structure and the physical parameters of the earths geologic layers from echo measurements. A complementary tool in relating seismic data to the earth's inhomogeneities is provided by seismic numerical models, which assume a subsurface structure and compute the seismic data which would be collected in a field survey, by solving the direct problem of exploration geophysics. This paper describes a vectorized and parallelized implementation of a two-dimensional seismic elastic model on the IBM 3090 VF Vector Multiprocessor. An implementation of a parallel seismic migration algorithm is then described. The paper also reports performance data for a vector/parallel implementation on the IBM 3090 of some typical seismic velocity estimation algorithms. The three problems chosen are representative of a wide class of geophysical computations, and the results summarized in this paper show their suitability for efficient implementation on the IBM 3090 Vector Multiprocessor; combined vector/parallel speedups in the range 15–25 are in fact observed.

Performance Evaluation Of The Boundary Lake South Triassic 'H' Pool

Abstract A reservoir engineering study of the Boundary Lake South Triassic "H " Pool was conducted to investigate the past performance and to forecast the performance potential of various depletion strategies. A detailed geological and petrophysical review of available data provided the characteristic distribution of reservoir properties. The history match phase of the study confirmed suspicions of fluid communication between the Triassic "H " Pool and a thief zone located above the reservoir. Model forecasts showed that the oil recovery could be increased by approximately 41% over the current development scheme with a program of infill drilling and modification to the injection patterns. Introduction The Boundary Lake South Triassic "H " Pool is located approximately 740 km northwest of Edmonton, Alberta. The pool was discovered in 1975 with the drilling of Well 10-10-84-12 W6M. The pool contains a 36 ° API crude with an initial reservoir pressure of 12 740 kPa. Initial estimate of the original oil-in-place (OOIP) was 2.91 × 106m3. A unit was formed which included the lands located in Sections 10 and 15, west-half of Section 11 and south-half of Section 22, Township 84, Range 12, west of the 6th Meridian. Oil production from the unit area commenced in 1975 under primary depletion. A pressure maintenance scheme by water injection was initiated in 1979. The void age replacement ratio was maintained close to 1.0 until oil production from the non-unit areas began in 1980. At the time of writing this paper, only one of the two non-unit operators is injecting water for pressure maintenance despite waterflood approvals given by the Energy Resources Conservation Board (ERCB) to both non-unit operators in 1985. As of the beginning of 1983, the unit production was approximately 70 m3 / day and has been experiencing a decline of close to 20% per year. A continuation of this rapid decline would have a serious negative impact on the oil recovery from the unit. As a result, Ultramar Oil and Gas Canada Limited, who is the unit operator, initiated a reservoir study for optimization of waterflood recoverable reserves within the unit. Geology The Boundary Lake member of the Triassic Charlie Lake formation comprises two sub-units in the area of the "H " Pool. The lower unit is composed mainly of shales and marl-anhydrite and does not constitute reservoir quality rock. This unit is characterized by four uniformly developed cycles which are easily recognized and was chosen as the structural and stratigraphic datum for reservoir mapping. A type log of the Boundary Lake member is shown in Figure 1. The upper sub-unit of the Boundary Lake member is the Boundary carbonate. The Boundary carbonate comprises four units, three cyclical units with uniform thickness of two to three metres each and an upper porous zone up to six metres thick as shown in Figure 2. The three lower cyclical units consist of a porous basal carbonate and an upper dense anhydrite or carbonate zone. The porous carbonates of the four cycles, lettered A to D from the basal unit, form the "H " Pool reservoir.

Reservoir Geology of Portion of Sailor Springs Consolidated Field and Its Impact on Primary and Secondary Production: ABSTRACT

The pool under study is part of the Sailor Springs Consolidated field, Clay County, Illinois. The 23 wells within the pool have produced more than 350,000 bbl of oil from the McClosky limestone (Mississippian-upper Valmayeran) since 1981. The average depth of the wells and the pay zone is 3,000 ft. The trap is predominantly stratigraphic in nature. Examination of core, thin sections, and geophysical logs indicates that the producing zone is composed of a complex series of oolitic bars. The individual bars are laterally discontinuous, flat bottomed, convex upward, and composed of oolitic skeletal grainstones that grade into dense skeletal wackestones and mudstones that act as reservoir seals. The bars are commonly subtle and easily overlooked. Several wells produce more than 100 BOPD from a zone less than 3 ft thick. Detailed mapping of total pore volume, total permeability, and facies cementation patterns is essential for successful field development and secondary waterflooding. These parameters have had a direct impact on primary and secondary production performance within the pool. Detailed reservoir mapping also reveals that the pool is subdivided into several reservoirs separated by reentrants that cut across the oolitic bars. End_of_Article - Last_Page 280------------

Application of Integrated Reservoir Analysis to Design of a Waterflood Project in Miocene LL3 Field, Lake Maracaibo, Venezuela: ABSTRACT

Integrated Reservoir Analysis is a procedure where interpreted stratigraphic and facies frameworks are combined with structural analysis to produce more accurate and appropriate reservoir maps. The result is a three dimensional interpretation of the thickness, shape, lateral extent, and internal distribution of porosity and permeability in individual reservoir units. The principal steps of the procedure are: (1) planning, (2) data gathering, (3) determination of the stratigraphic framework, (4) determination of the facies framework, (5) structural analysis, (6) data manipulation, and (7) mapping. The stratigraphic framework is developed by combining pattern correlation techniques with knowledge about the influence of specific facies on stratigraphic patterns. A network of cross sections are designed utilizing correlation grain and/or depositional strike. Correlations on these sections develop a framework of horizons which ideally will isolate, in a stratigraphic envelope, individual reservoir units resulting from a unique depositional episode. The facies framework results from environmental facies analysis and the use of electric log facies. The proper identification of reservoir facies is required for the mapping of reservoir geometry and the determination of the internal distribution of porosity and permeability. Facies-biased contouring techniques and the lateral extension of facies relationships along cross sections were used in the planning and design of a waterflood project by Lagoven, S.A., in fluvial-deltaic clastics of the Miocene La Rosa Formation in the LL3 field, Lake Maracaibo, Venezuela. Important clastic reservoir facies recognized in cores were (1) stream-mouth-bar, (2) distributary-channel-fill, and (3) fluvial point-bar deposits. These environmental facies often occurred in various combinations in deltaic obes and displayed the electric log shape of the deltaic couplet. Characteristic electric log shapes of specific reservoir facies were an essential part of the study. The pilot waterflood was designed to inject into stream-mouth-bar facies and withdraw from centrally located distributary-channel-fill deposits with their better porosity and permeability. Critical to the design and subsequent performance of the waterflood project were (1) the distribution of porosity within the various reservoir facies, and (2) the occurrence, attitude, and lateral distribution of clay laminations within the lower stream-mouth-bar and upper fluvial deposits. The influence of clay laminations in lower stream-mouth-bar facies was particularly critical to waterflood performance. They were deposited on the distal slope of the stream-mouth-bar at a slight angle to the rock unit boundaries and therefore could mask parts of the reservoir from waterflood treatment. The amoun of masking was determined by calculations from facies geometry. After one year's operations, radioactive tracers indicate that the flood is operating as designed only at a reduced rate--probably as a result of the clay laminations. Figure Integrated Reservoir Analysis can be useful not only to production operations such as waterflood projects, infill drilling, recompletions, and reserve estimates, but a similar procedure could also be applied to exploration activity in mature areas with plentiful log data. End_of_Article - Last_Page 555------------

Computer Analysis of Early Well Logs: ABSTRACT

Geophysical well logs have been recorded in wells drilled for petroleum and natural gas since the early 1930s. These, largely hard-copy records, comprise the greatest pool of factual subsurface information on producing reservoirs and on new prospects that were either not economic or overlooked when the wells were drilled originally. As such, they are a valuable exploration tool but are difficult to use because of the incompatibility of recorded information both within and between wells. Early well logs display a wide range of curve types with a End_Page 541------------------------------ remarkable variety of calibration and depth scales. Visual analysis with well-to-well continuity is almost impossible with the logs in their original form. Fortunately, the hard-copy logs can be digitized and computer-processed to produce standardized logs that are amenable to accurate analysis for contained hydrocarbons and the spatial mapping of potential reservoirs. The process does require the services of a petroleum geologist to determine the log response to clean formations within each well. However, once the individual curve parameters are determined, the computer can carry out the detailed computations for the display and isolation of all potential reservoirs. Examples of logs from New York state indicate that many gas-bearing reservoirs remain to be exploited. End_of_Article - Last_Page 542------------

Exploration and Development of Lobate Back-Barrier Facies Sandstones: ABSTRACT

Lobate back-barrier sandstones deposited as washover fans and floodtidal deltas are major reservoirs for stratigraphically trapped hydrocarbons on the Gulf Coast. Understanding back-barrier facies relationships can improve the efficiency of exploration and development in three ways: (1) recognition of their irregular to lobate geometry (rather than simply linear parallel to depositional strike) allows more accurate predictive reservoir mapping, (2) detailed correlation and mapping of each individual wedge of reservoir provides more precise determination of the updip pinch-out, and (3) resistive zones on electric logs, representing coals or tight sandstones, can be used as adjacent clues to the nearby presence of a back-barrier reservoir. Application of these concepts in south Texas led to extension of the previously abandoned Draper field. End_of_Article - Last_Page 1464------------

Integrated Stratigraphic and Seismic Approaches to Reservoir Mapping, Mibale Field, Offshore Zaire: ABSTRACT

Located on a north-northwest-trending growth fault block on the continental shelf of offshore Zaire, the Mibale field produces medium grade oil from the Cretaceous Pinda Formation and is sealed by calcareous shales of the overlying Iabe Formation. An integrated geologic-geophysical study of the field resulted in detailed characterization and mapping of the Pinda reservoir, from which locations for future development and water injection wells have been selected. The Albian-Cenomanian carbonates and terrigenous clastics of offshore Zaire form a transgressive sequence comprised of landward thickening supratidal and intertidal deposits, and seaward thickening subtidal deposits. In the Mibale field major lithofacies are: (1) supratidal--thick-bedded sandstones and dolomites, with minor shales and anhydrites; (2) intertidal--thin-bedded shales, dolomites, and sandstones with minor anhydrites; and (3) subtidal--(a) thick-bedded dolomites and sandstones; (b) dolomites; and (c) lime wackestones to packstones containing various skeletal and non-skeletal grains. Cores show that the sandstones and dolomites have better permeabilities than the limestones. The areal distribution of permeable reservoirs is controlled by facies configuration, growth-fault movements, and secondary dolomitization. Within a single facies, thickness variations greater than 300 ft (91 m) occur. Because of the unpredictable thickness variations, seismic inversions (pseudo-sonic logs) were used to define the subsurface configuration of depositional and diagenetic facies across the field. Inversions were carefully calibrated to sonic logs and used for direct interpretation of lithology in areas lacking well control. Structure and gross pay maps, as well as lithologic and petrophysical characteristics of reservoir facies, enabled engineers to develop a three-dimensional simulation model of the field. End_of_Article - Last_Page 996------------

Evaluation of Volume of Entrained Methane in Deep, Tertiary Sandstone Reservoirs Along Onshore Texas Gulf Coast: ABSTRACT

Volume of methane dissolved in formation waters in Tertiary sandstone reservoirs is directly related to sandstone volume and porosity, formation temperature and pressure, and salinity of formation fluids. Volume of potential reservoirs, located on regional cross sections below 8,000 ft (2,438 m; approximate base of oil production), was determined from isopach and net-sandstone maps of each formation. Plots show variation of formation pressure, temperature, and salinity with depth in all wells on cross sections. Porosity was determined by whole-core analyses, supplemented by sonic and resistivity logs. The Texas Gulf Coast was divided into 24 subdivisions for detailed reservoir mapping and calculation of methane resource. Subdivision boundaries were defined by structural provinces and by major fault zones. Subdivision 2, an area of 5,300 sq mi (13,727 sq km) along the lower Texas coast, illustrates methods for evaluating the amount of methane dissolved in formation waters. In the lower Frio Formation, an in-place resource estimate of 61 quads (1 quad = 1 Tcf) was calculated by averaging parameters in 1,000-ft (305 m) intervals, a method similar to that used by the U.S. Geological Survey. A comparable estimate, 56 quads, was calculated by using parameters averaged for the entire lower Frio Formation. Total in-place methane of 144 quads for all formations in Subdivision 2 was determin d using the latter method. Similar evaluation of all subdivisions will define the entrained methane resource of the onshore Texas Gulf Coast. End_of_Article - Last_Page 699------------