Fluvial-deltaic Reservoirs Research Articles

Application of seismic sedimentology in characterization of fluvial-deltaic reservoirs in Xihu sag, East China Sea shelf basin

The fluvial-deltaic reservoirs of the Oligocene Huagang Formation in the Xihu sag of the East China Sea shelf basin reflect rapid lateral change in sedimentary facies and poor morphology of conventional slice attributes, which bring difficulties to the reservoir prediction for subsequent exploration and development of lithologic reservoirs. The traditional seismic sedimentology technology is optimized by applying the characteristic technologies such as frequency-boosting interpretation, inversion-conventional −90° phase shift joint construction of seismic lithologic bodies, nonlinear slices, paleogeomorphology restoration, and multi-attribute fusion, to obtain typical slice attributes, which are conducive to geological form description and sedimentary interpretation. The Huagang Formation developed three types of sedimentary bodies: braided river, meandering river and shallow water delta, and the vertical sedimentary evolution was controlled by the mid-term base-level cycle and paleogeomorphology. In the early–middle stage of the mid-term base-level ascending cycle, braided channel deposits were dominant, and vertical superimposed sand bodies were developed. In the late stage of the ascending half-cycle and the early stage of the descending half-cycle, meandering river deposits were dominant, and isolated sand bodies were developed. In the middle–late stage of the descending half-cycle, shallow-water delta deposits were dominant, and migratory medium–thick sand bodies were developed. Restricted paleogeomorphology controlled the sand body distribution, while non-restricted paleogeomorphology had little effect on the sand body distribution. Based on reservoir characterization, the fault sealing type and reservoir updip pinch-out type structural lithological traps are proposed as the main directions for future exploration and development in the Xihu sag.

Ancient delta deposits in the Ivie Creek area, Ferron Sandstone member of the Mancos Shale, western San Rafael Swell, east-central Utah

In contrast to the beautiful array of colorful layers and spectacular cliffs of the Triassic and Jurassic (251 to 148 million years ago [Ma]) sections in the San Rafael Swell of east-central Utah, most of the Upper Cretaceous (96 to 86 Ma) Mancos Shale produces a drab, barren landscape. However, lying within the Mancos, the Ferron Sandstone, is the most studied unit in the San Rafael Swell. The Ferron has world-class outcrops of rock layers deposited near the shorelines of a sinking, fluvial- (stream) dominated delta system. Along the west flank of the San Rafael Swell, the 80-mile-long (130 km) Ferron outcrop belt of cliffs and side canyons (e.g., the Coal Cliffs, Molen Reef, and Limestone Cliffs [not actually limestone, just misnamed]) provides a three-dimensional view of vertical and lateral changes in the Ferron’s rock layers (facies and sequence stratigraphy), and, as such, is an excellent model for fluvial-deltaic oil and gas reservoirs worldwide (e.g., Chidsey and others, 2004).

The rebirth of Papua New Guinea

Gas was first discovered by drilling activity in Papua New Guinea in 1956. Nevertheless it was almost 60 years later when the Exxon-operated PNG LNG Project became the first large-scale commercialisation of the country’s prolific gas resources, with export of the first LNG shipment in May 2014. The proven fluvial-deltaic Late Jurassic, early Cretaceous and Tertiary carbonate reservoirs are prolific petroleum producers. When combined with the high-quality liquid-rich nature of the gas and the onshore location, this has resulted in PNG’s LNG projects being some of the lowest cost and most profitable LNG projects globally. The success of the PNG LNG Project along with the substantial identified existing and yet-to-find gas resources has resulted in a recent resurgence of exploration interest in PNG as companies look to expand and capitalise on the country’s developing position as a globally significant LNG supplier. In 2015 Oil Search undertook a whole of country review using its extensive database and in-country knowledge. This study incorporated all well, seismic, surface, remote sensing, production and development data, and has resulted in a detailed understanding of the play distribution and risk ranking, and importantly delineated a number of potentially material new play types in the country. Approximately 4.8 billion barrels of oil equivalent recoverable resources (2P and 2C) have been discovered in PNG to date, of which approximately 85% is gas. The countrywide regional study and common risk segment analysis by Oil Search (2015) established that PNG potentially contains an additional seven billion barrels of oil equivalent resource still to be discovered. The majority of this volume is expected to be gas, estimated to be in the order of 40 trillion cubic feet and 550 million barrels of undiscovered prospective resources. Oil Search has been active in all of PNG’s known petroleum basins since 1929 and built a substantial database. Pool-size distributions and a countrywide prospects and leads inventory suggest discovery of new giant fields is likely and of sufficient scale to support future LNG projects. Six sub-basins contain proven petroleum plays that are predicted to extend into under-explored areas. These areas represent the future petroleum exploration frontier in PNG.

Integrating phase into the visualization of spectral decomposition attributes

Much of the world’s conventional oil and gas production comes from fluvial-deltaic reservoirs. The ability to accurately interpret the architectural elements comprising these systems greatly reduces the risk in exploration and development in these environments. We have evaluated methods for using spectral decomposition attributes to improve the visualization in fluvial-deltaic environments using data from the Middle Pennsylvanian age Red Fork Formation of Oklahoma. We determined how spectral phase and magnitude attributes can be effectively combined using an hue-saturation-value color map to produce images that have considerable interpretational value. Incorporating our methods in the interpretation process has the potential to improve the exploration and development in these systems.

A review and technical summary of the AAPG Hedberg Research Conference “Variations in fluvial-deltaic and coastal reservoirs deposited in tropical environments”

This conference addressed the question of whether fluvial-deltaic and coastal reservoirs deposited in tropical environments exhibit different facies relationships than those of temperate systems. Although this conference concluded that there is nothing distinctly unique about clastic depositional systems in tropical environments, the magnitude of the processes that operate in these environments differs from nontropical environments. Because these processes influence the distribution of reservoir and nonreservoir facies, geologists need a fundamental understanding of the contrasts between tropical and nontropical environments when interpreting subsurface data and building depositional models. We highlight the following features that seem to be more favorably developed in tropical environments. (1) Many tropical systems exhibit complexly stacked channel and overbank deposits. Although some fluvial deposits have the classic meandering pattern, seismic attributes indicate that many tropical channel belts are anastomosing systems that intersect each other at large angles. Subsurface correlation without the benefit of three-dimensional seismic data is all but impossible. (2) In contrast to deltaic deposits in temperate climates, which exhibit high sand/shale ratios near the delta with sand content decreasing basinward and laterally away from the delta, tropical deltas tend to be muddier near the river mouth and have cleaner, well-sorted sand bodies better developed laterally in the shoreface and shallow-marine environments. (3) Modern tropical coasts are home to large estuarine environments dominated by mangroves. Estuarine deposits interpreted in the subsurface and observed in outcrop are sand rich, yet little sand is observed in modern mangrove estuaries. Mangrove estuarine deposits also contain abundant organic-rich layers, thin coals, and dispersed organic matter that may favor the development of a self-sourced petroleum system. The flushing of these systems during relative low stands may be an important mechanism not only for reservoir development, but also for dispersing organic matter into deep-water systems. This conference was dominated by discussions of depositional systems in Southeast Asia, which have a strong tectonic overprint. Further research on tropical systems from the South American and African conjugate margin could provide important tests for the ideas highlighted above.

Trade-off between stability and bias in a history match problem using smoothness constraint

The history matching (HM) hydrocarbon reservoir inverse problem is ill-posed because its solution may be non-unique or unstable. Variability of acceptable solutions around local minima may be very large. Nonetheless, it is a common practice in HM to find a unique reservoir model, although there is no guarantee that this model represents the geology. Qualitative geological information is generally not considered because of difficulties to express them mathematically. Here, we incorporate ‘smoothness’ in the spatial variation of physical properties as an example of a geological qualitative constraint that can be mathematically incorporated in an objective function also honouring the data. The constraint is valid if lateral continuity exists (e.g. fluvial-deltaic siliciclastic reservoirs). We mean smoothness by conditioning the permeability and/or porosity difference between adjacent grid blocks to be small. We use a synthetic 2D water–oil model and a solution search technique allowing characterising both the optimum solution and its variability. The smoothness constraint reduces the variance of the estimates by introducing bias in the solutions still preserving good match. The key point to achieve an optimum trade-off between stability and data match is the tuning of the parameter, controlling the relative importance of the constraint in the objective function. The smoothness constraint cannot be applied to all reservoirs. However, we corroborate the idea that a ‘tool box’ of HM can be designed; each ‘tool’ can incorporate a different constraint. Therefore, the interpreter can judiciously choose a specific tool from this tool box according to the adherence of its constraint to the particular reservoir being studied.

Abstract: Anatomy and Oil Recovery from Fluvial-Deltaic Reservoirs in the Cottage Grove Sandstone, Lake Blackwell Field, Central Oklahoma

Abstract: Anatomy and Oil Recovery from Fluvial-Deltaic Reservoirs in the Cottage Grove Sandstone, Lake Blackwell Field, Central Oklahoma

Heterogeneity of Fluvial-Deltaic Reservoirs in the Appalachian Basin: A Case Study from a Lower Mississippian Oil Field in Central West Virginia

Since discovery in 1924, Granny Creek field in central West Virginia has experienced several periods of renewed drilling for oil in a fluvial-deltaic sandstone in the Lower Mississippian Price Formation. Depositional and diagenetic features leading to reservoir heterogeneity include highly variable grain size, thin shale and siltstone beds, and zones containing large quantities of calcite, siderite, or quartz cement. Electrofacies defined through cluster analysis of wireline log responses corresponded approximately to facies observed in core. Three-dimensional models of porosity computed from density logs showed that zones of relatively high porosity were discontinuous across the field. The regression of core permeability on core porosity is statistically significant, and differs for each electrofacies. Zones of high permeability estimated from porosity and electrofacies tend to be discontinuous and aligned roughly north-south. Cumulative oil production varies considerably between adjacent wells, and corresponds very poorly with trends in porosity and permeability. Original oil in place, estimated for each well from reservoir thickness, porosity, water saturation, and an assumed value for drainage radius, is highly variable in the southern part of the field, which is characterized by relatively complex interfingering of electrofacies and similar variability in porosity and permeability.

Determining Between-Well Reservoir Architecture in Deltaic Sandstones Using Only Well Data: Oligocene Frio Formation, Tijerina-Canales-Blucher Field, South Texas: ABSTRACT

ABSTRACT Accurate prediction of compartment architecture and intracompartment heterogeneity is necessary to locate and recover the estimated 15 billion barrels of mobile oil remaining in U.S. fluvial-dominated deltaic reservoirs. Complex architecture and rapid 1ateral variability in such reservoirs complicate subsurface prediction, particularly in mature fields where well logs are the only available subsurface data. A genetic-stratigraphy-based methodology has been developed that improves between-well prediction of deltaic reservoir architecture and, thus, reduces risks associated with infill-drilling. In the area of Tijerina-Canales-Blucher (T-C-B) field, which lies on the northeast margin of the Oligocene-age Norias delta, the productive 3rd-order Lower Frio unit was subdivided into eight 4th-order genetic units. Delta-front positions were identified on the basis of regional and subregional cross sections. The 4th-order units (30 to 80 ft thick) were subdivided into two to five 5th-order units (10 to 30 ft thick). Log patterns and net sandstone maps were used to identify facies, which include (1) distributary channels (up to 25 ft thick, 8,000 ft wide, and commonly narrower than 40-acre well spacing), (2) mouth bars (up to 15 ft thick, ranging in size from 40 to 640 acres in area, commonly 5,000 ft wide), and (5) washover fans (up to 10 ft thick, and 7,000 ft wide). Many reservoir compartments, including the prolific 21-B interval, contain a significant degree of stratigraphic trapping caused by updip pinchout of delta front or washover sandstones or convex-updip segments of meandering distributary channel sandstones. The methodology and results of this Study are directly applicable to other Gulf Coast fluvial-deltaic reservoirs in the Frio Formation and Wilcox Group, as well as to deltaic reservoirs throughout the U.S. The general methodology should be applied to develop remaining reserves in mature fields where geophysical data are lacking. Further study of T-C-B reservoirs may include stratigraphic analysis of 3-D seismic data and infill drilling to confirm between-well interpretations. End_of_Record - Last_Page 475-------

Characterization of Heterogeneity Style and Permeability Structure in Fluvial-Deltaic Reservoirs Deposited in a Low-Accommodation Setting: ABSTRACT

Because of the worldwide importance of resources in fluvial-deltaic reservoirs, a consortium of companies is funding research at The University of Texas aimed at reservoir characterization of fluvial-deltaic depositional systems. The goals of this industrial associates program are to develop an understanding of sandstone architecture and permeability structure in a spectrum of fluvial-deltaic reservoirs and to translate this understanding into more realistic, geologically constrained reservoir models. Our approach is to quantify the interrelationships among sequence stratigraphy, depositional architecture, diagenesis, and permeability structure through detailed outcrop characterization. The current project focus is the Lower Cretaceous Fall River Formation, a deltaic system deposited in a low-accommodation setting in South Dakota and Wyoming, USA. The Fall River is exposed around the margins of the Black Hills Uplift and produces oil and gas in the adjacent Powder River Basin. Extensive field work established the Fall River as a storm- and tide-dominated deltaic depositional system in which the shallow-water marine deposits were locally removed by valley incision during periods of base-level fall. Valley-fill sandstones, which form the most productive Fall River reservoirs, were deposited during base-level rise and grade upward from low-sinuosity fluvial to estuarine deposits. Permeability structure in marine and valley-fill deposits was determinedmore » from >5,000 minipermearneter data points in vertical and horizontal transects. In this low - accommodation setting, unconformity-bounded valley fills are flow units, and pronounced lateral changes in permeability occur where valley fills of different character am juxtaposed. The interpretations of the Fall River that were developed on outcrop air being used to guide characterization of Buck Draw field, a Fall River field with an outstanding suite of cores and reservoir production data.« less

Merging Basic Science and Applied Reservoir Characterization Research: An Effective Approach for Assisting Industry in Field Optimization for Incremental Recovery of Oil and Gas: ABSTRACT

Using a dual approach of basic research and applied field-optimization studies, Bureau geoscientists are helping industry maximize the recovery of oil and gas in stratigraphically complex reservoirs. Basic research on reservoir architecture is supported by two industrial associates programs that are evaluating siliciclastic reservoirs (Characterization of Heterogeneity Style and Permeability Structure in a Sequence Stratigraphic Framework in Fluvial-Deltaic Reservoirs) and carbonate reservoirs (Characterization of Carbonate Reservoirs) by combining outcrop analogs of major reservoir types and subsurface reservoir studies. In these two programs, high-frequency sequence stratigraphic analysis--along with analyses of rock fabric, permeability structure, bounding surfaces, and reservoir stacking patterns-is being evaluated in order to simulate interwell heterogeneity and its effect on fluid flow. Results are transferred to more than 25 companies worldwide through workshops, field courses, mentor programs, reports, journal articles, preprints and data sets. Research results from oil and gas field-optimization studies in North America, South America, and Australia have combined play assessment, detailed sequence stratigraphic and depositional-system analyses, and reservoir characterization to strategically deploy technology and achieve reserve growth in fields that have been producing for as long as 50 years. Geocellullar modeling and three-dimensional visualization of geologic, petrophysical, and engineering data are critical to establishing accurate estimatesmore » of original oil in place and remaining mobile oil. Reservoir settings, including tide-dominated deltaic, shelf, fluvial, and lacustrine-delta depositional systems, have been targeted for new infield wells, recompletions, and step-out locations. Proven reserves of more than 450 million barrels of remaining mobile oil have been identified in one field optimization study.« less

Modeling Reservoir Attributes and Estimating Additional Hydrocarbon Potential for Redevelopment in Fluvial-Deltaic Reservoirs: An Example from the Frio Fluvial-Deltaic Sandstone Play in South Texas: ABSTRACT

Modeling Reservoir Attributes and Estimating Additional Hydrocarbon Potential for Redevelopment in Fluvial-Deltaic Reservoirs: An Example from the Frio Fluvial-Deltaic Sandstone Play in South Texas: ABSTRACT

Geological and Petrophysical Characterization of the Ferron Sandstone in Utah, for 3-D Simulation of a Fluvial-Deltaic Reservoir: ABSTRACT

Fluvial-deltaic reservoirs contain the largest developed domestic oil reserves, and due to the high degree of reservoir heterogeneity, the largest amount of unrecovered oil of any type of developed reservoirs. The excellent outcrops of the fluvial-deltaic Ferron Sandstone Member of the Cretaceous Mancos Shale make the Coal Cliffs in east-central Utah a world-class area to study reservoir heterogeneity. With funding from the U.S. Department of Energy, a multidisciplinary team is developing a 3-D reservoir model for the Ferron Sandstone. Our work to date has consisted of: (1) determining geological and petrophysical properties of significant lithofacies, (2) developing new field methods, and (3) interpreting the stratigraphy regionally and within case-study areas. Numerous sections have been measured, described, and correlated to interpret of the sequence stratigraphy and lithofacies. Lithologic and paleocurrent data were entered into a database to construct statistical models, strip logs, and lithofacies maps. Several gamma-ray traverses were undertaken primarily for stratigraphic correlation. Numerous core plugs were obtained from various lithofacies for porosity, permeability, and other types of reservoir analyses. Permeability transects were completed across the proximal, middle, and distal portions of the delta-front sandstones. Six core holes were drilled in the central case-study area in a pattern similar tomore » that drilled in an oil field. The data collected will be integrated into a 3-D representation of spatial variations in porosity, storativity, and tensorial rock permeability at a scale appropriate for inter-well to regional-scale reservoir simulation. Results could improve management of similar reservoirs worldwide through proper infill and extension drilling strategies, reduction of economic risks, increased recovery, and more reliable reserve calculations.« less

Reservoir Architecture and Permeability Characteristics of Fluvial-Deltaic Sandstone Reservoirs, Frio Formation, Rincon Field, South Texas: ABSTRACT

Reservoir Architecture and Permeability Characteristics of Fluvial-Deltaic Sandstone Reservoirs, Frio Formation, Rincon Field, South Texas: ABSTRACT

High-Resolution Sequence Stratigraphy: The Key to Identifying Compartment Styles in Frio Formation Fluvial-Deltaic Reservoirs, T-C-B Field, Jim Wells County, Texas: ABSTRACT

High-Resolution Sequence Stratigraphy: The Key to Identifying Compartment Styles in Frio Formation Fluvial-Deltaic Reservoirs, T-C-B Field, Jim Wells County, Texas: ABSTRACT

Application of Sequence Stratigraphy to the Prioritization of Incremental Growth Opportunities in Mature Reservoirs: An Example from Frio Fluvial-Deltaic Sandstones, T-C-B Field, South Texas: ABSTRACT

ABSTRACT The U. S. Department of Energy has identified mature fluvial-deltaic reservoirs as being the highest priority reservoir type for near-term domestic reserve-growth potential. Detailed characterization studies to locate the estimated 15 billion barrels of mobile oil remaining in these reservoirs must focus first on those reservoirs with the greatest potential if they are to be economically successful. In quick-look analyses that estimate reserve-growth potential, stratigraphic heterogeneity is the most difficult factor to evaluate. Through detailed study of Frio Formation upper delta-plain fluvial reservoirs in T-C-B field, south Texas, and by using outcrop observations from the Ferron Sandstone, central Utah, a model has been developed that relates stratigraphic heterogeneity to rate of accommodation during deposition, a feature that varies predictably throughout a depositional cycle. Upper delta-plain fluvial channelbelts deposited at the base of a depositional cycle, during periods of low accommodation, are narrow, few in number, and relatively internally homogeneous. As a result, they are not effectively contacted by rigid well patterns but can drain large reservoir volumes through individual completions. Because they are commonly isolated within floodplain mudstones, they possess the potential for stratigraphically trapped accumulations away from the structural crest. In contrast, channelbelts deposited at the top of a depositional cycle, during periods of high accommodation, are broad and internally heterogeneous. Despite appearing laterally continuous, they are extensively compartmented by high volumes of low-permeability siltstones and shales, as well as mudclast-rich lag deposits that drape channel boundaries and limit fluid flow at channel-on-channel contacts. AU other factors being equal, these channelbelt types possess the greatest reserve-growth potential because past completions have only contacted small reservoir volumes. This accommodation-based model for reservoir heterogeneity in upper delta-plain fluvial settings represents the first in a new generation of reservoir models. Similar models developed for reservoirs deposited in the spectrum of depositional settings will improve the effectiveness and increase the use of detailed reservoir characterization studies, resulting in the improved identification and recovery of oil and gas remaining in mature reservoirs throughout the United States.

Targeting the Unrecovered Oil Resource in Mature Field Areas: An Example from Oligocene Frio Fluvial-Deltaic Reservoirs of South Texas: ABSTRACT

Targeting the Unrecovered Oil Resource in Mature Field Areas: An Example from Oligocene Frio Fluvial-Deltaic Reservoirs of South Texas: ABSTRACT

The Boscan Field -- Venezuela's Giant Stratigraphic Trap With A Complex History

The Boscan Field lies in the northwest quadrant of the Maracaibo Basin, Venezuela. It is a large stratigraphic trap that has produced more than 800 million barrels of heavy oil since 1946, principally from Eocene fluvial-deltaic clastic reservoirs. These are faulted, folded, and are truncated by a pre-Oligocene unconformity. The geochronology of the Boscan Field has been interpreted as follows: (1) Eocene fluvial-deltaic deposits prograded east and southeast across the Boscan Field Area. A subsiding area northeast towards the State of Falcon received tremendous quantities of Eocene fine grained organic-rich deposits in pro-delta and deep water environments. (2) The Eocene shales in the deep basin began to generate hydrocarbons which migrated towards the west and southwest. (3) In the Boscan Area, Eocene deposits were folded, faulted, and tilted towards the east and southeast and an active fluvial flood-plain truncated the deposits resulting in subcrop bands that form the pre-Oligocene unconformity topographic surface. (4) Hydrocarbon generation and migration continues in the deep basin. (5) Oligocene fluvial sandstones deposited on the underlying unconformity often cut into and are in direct contact with truncated Eocene sandstones. This allowed some oil to leak up into the Oligocene deposits. (6) By the end of themore » Oligocene, migration of oil into the gross Boscan trap was well underway. The east-west and northwest southeast trending fluvial conduits carried the oil out of the Eocene basin northwestward until it was blocked by reservoir truncation on the west and northwest sides of the Boscan Field. (7) Regional subsidence of the Maracaibo Basin has caused the Boscan reservoirs to dip towards the southwest resulting in realignment of fluids. The present-day Boscan Field is a composite of stacked, linear, faulted, and truncated reservoir segments. They may be isolated or interactive with other segments and form a complex [open quotes]plumbing[close quotes] tangle.« less

Re-Exploration of Cratonic Basins Using Passive-Margin Sequence-Stratigraphic Concepts: Examples from Upper Paleozoic Rocks, Eastern Margin, Midland Basin: ABSTRACT

Use of 5000 well logs and extensive outcrop information with a 22,000-mi/sup 2/ test region on the eastern margin of the Midland basin permitted delineation of 16 probably third-order type 1 depositional sequences. Sandstone-isolith maps of siliciclastic highstand and lowstand systems tracts show that most structural traps produce from highstand fluvial-deltaic reservoirs, but most stratigraphic traps discovered to date occur within lowstand depositional systems, principally incised valley fills and basin-floor fans. Hydrocarbons are rarely trapped in retrogradational (transgressive) systems tracts. Maps of lowstand tracts refocus attention on reservoirs that can be predicted to exist basinward of preexisting shelf edges. A basinward shift of exploration emphasis from incised valley-fill reservoirs to other lowstand elements - such as basin-floor fans, canyon and leveed-channel fills, and lowstand progradational deltaic wedges - could lead to plays where lenticular reservoir sandstones and marine-condensed source and seal shales exhibit the optimum conditions for pinch-out traps.

Application of Facies Analysis to Improve Gas Reserve Growth in Fluvial Frio Reservoirs, La Gloria Field, South Texas: ABSTRACT

Geologically based infill-drilling strategies hold great potential for extension of domestic gas resources. Traditional gas-well drilling and development have often assumed homogeneous and continuous reservoirs; uniform gas-well spacing has resulted in numerous untapped reservoirs isolated from other productive sand bodies. Strategically located infill wells drilled into these undrained reservoirs may ultimately contact an additional 20% of original gas in place in Texas gas fields. Tertiary formations in the Texas Gulf Coast commonly exhibit multiple fluvial and fluvial-deltaic reservoirs that contain vertical and horizontal permeability barriers. For example, the Frio La Gloria field (Jim Wells and Brooks Counties, Texas) contains isolated and compartmentalized reservoirs that can be related to the irregular distribution of heterogeneous facies. Net-sand and log-facies maps in areas of dense well spacing delineate relatively continuous pay defined by lenticular point-bar and channel-fill deposits 1,500-2,500 ft wide. These point-bar deposits are flanked laterally by sand-poor levee and splay facies that isolate the reservoirs into narrow, dip-elongate bands.