Distributary Sandstones Research Articles

Transgressive successions of the Mahakam Delta Province, Indonesia

Abstract Sedimentological and stratigraphic analysis of outcrops and subsurface data in the Mahakam Delta province, combined with concepts of transgressive sedimentary processes derived from a study of the modern Mahakam Delta, indicate that a significant portion of the palaeo-Mahakam Delta succession was deposited during transgressive phases. Some of the transgressive successions resulted from major transgressions and are regionally extensive, but many reflect small, short-lived transgressions within dominantly progradational phases. Two sandstone facies with significant reservoir potential are common within the transgressive successions. Backfilled distributary sandstones occur in outcrop as 10–20 m thick, fining-upward channel sands that become more marine upward and in the subsurface as elongate, coastline-perpendicular sand bodies with a back-stepping stratigraphic architecture and highly variable thickness. Transgressive shoreline sandstones also fine upward, but they are shoreline-parallel, their lateral extent is controlled by the pre-transgression delta morphology and their thickness depends on the rate of relative sea-level rise. Both types of transgressive sandstone can be difficult to distinguish from specific progradational sandstones with wireline log data, especially when they occur within predominantly progradational successions. However, their sand body geometries, volumetrics and connectivity are much different from the stratigraphically adjacent sandstones deposited during progradational phases of deltaic deposition.

Outcrop Quantification of Flow-Unit and Seal Properties and Geometries, Ferron Sandstone, Utah

River-dominated deltaic sandstones of the Upper Cretaceous Ferron Member of the Mancos Shale provide the opportunity to quantify the 3-D architecture and permeability structure of fluvial, deltaic, and shorezone sandstones that are analogous to large oil and gas reservoirs of the Gulf Coast. Current investigations, working within a previously established stratigraphic framework, are aimed at quantifying flow-unit character as well as delineating bounding and sealing surfaces that cause inefficient drainage and the abandonment of conventionally recoverable hydrocarbons in reservoir analogs. Measured sections at 1-mi spacings establish the facies framework within which windows are selected for detailed study. Average permeabilities are highest in distributary-channel facies. Distributary sandstones are, however, also characterized by the highest degree of permeability heterogeneity. Lateral permeability heterogeneity, a response to lateral accretion and channel-flooring scours, is pronounced. In contrast, associated shoreface sandstones have lower average permeabilities, simple upwardly increasing permeability profiles, and a high degree of lateral continuity. Vertical heterogeneity, provided by laterally continuous mud layers, is dominant. Communication between distributary channel and associated shoreface sandstones is likely to be hindered by low-permeability (less than 5 md) zones that consistently occur at the base of channel sandstones and along the toesets of lateral accretion surfaces.

Paleogeographic and Stratigraphic Constraints on Lower Cretaceous Mesa Rica Delta System, East-Central New Mexico: ABSTRACT

The Mesa Rica Sandstone of east-central New Mexico undergoes facies transition from nonmarine fluvial distributary channels to fluvial-dominated deltaic strata over an outcrop distance of approximately 40 km. The underlying Tucumcari Shale contains equivalent prodelta deposits, and the overlying Pajarito Shale represents the delta-plain environment. The delta contains two major progradational lobes, each directly traceable into its correlative distributary channel system. Paleoflow within the older distributary system is toward the south-southwest (206/sup 0/), and progradation of its corresponding delta lobe is also in that direction (215/sup 0/). Eastward shift in fluvial transport (to 107/sup 0/) resulted in delta-lobe abandonment and established a younger lobe that prograded toward the east-southeast (106/sup 0/). Mesa Rica distributary sandstones correlate westward with alluvial strata of unknown age that have been assigned to the lower braided interval of the Dakota Formation or to the Mesa Rica Sandstone. Results of their study clearly link these fluvial deposits to the Mesa Rica delta front, which is of Albian age based on the presence of the index ammonoid Mortoniceras equidistans. They support assignment of these fluvial strata to the Mesa Rica Sandstone and reserve use of the name Dakota Formation for the overlying Cenomanian marine transgressive sandstone.

Penecontemporaneous Facies Relations in Pennsylvanian (Desmoinesian) Deltas of Southwestern Indiana: ABSTRACT

Lateral and vertical sedimentologic and paleoecologic analyses of rocks and fossils of the lower part of the Dugger Formation (Pennsylvanian, Desmoinesian) in Indiana has delineated the following deltaic sub-environments: (1) distributary sands, (2) upper-, middle-, and lower-interdistributary estuaries, (3) delta-plain lakes, and (4) swamps. In this part of the Illinois basin, terrigenous influx was from the northeast across an extremely low depositional gradient. Because of high surface area to depth ratio, current and wave energies of this shallow epicontinental sea were diminished. Thus, differential compaction rather than marine reworking dominated the deltaic destructive phase in this area. Differential compaction resulted in accumulation of localized anomalously th ck sequences. The thickened units occur directly adjacent to penecontemporaneous distributary sandstones because of the greater degree of syndepositional subsidence in these areas. The dominance by differential compaction produced an unusual situation wherein a slow marine transgressive or deltaic abandonment stage was followed by a rapid marine regressive or deltaic progradational stage. Vertical and lateral changes in body- and trace-fossil communities reflect the encroachment of marine conditions and the relatively sudden onset of freshwater deposition. Maximum marine inundation of the area coincided with carbonate deposition. Maximum water depth in the area is estimated to have been less than 80 ft (25m) based on lateral relationships of interdistributary lithofacies with distributar sandstone and delta-plain sediments. The marine-influenced lithology deposited in shallowest water was black shale, which grades downward into coal and upward into calcareous gray shale and limestone. Lateral facies gradation indicates that these divergent lithotypes were produced penecontemporaneously at the delta margin. End_of_Article - Last_Page 283------------

Cyclic Sedimentation Within Point Lookout Formation (Upper Cretaceous)--A Model for Strandline Progradation and Sandstone Distribution: ABSTRACT

The regressive marine Point Lookout Sandstone in the southeastern San Juan basin, New Mexico, was deposited under conditions of relatively low wave energy, moderate tidal strength, and locally moderate fluvial input. Two idealized coarsening-upward vertical profiles characterize the preserved lithologic record. The first profile, documenting strike-aligned sandstones dominated by wave processes, is composed of the upward progression: offshore mudstone--offshore transition mudstone to sandstone--lower shoreface sandstone--upper shoreface sandstone--estuarine sandstone. The second vertical profile, reflecting local fluvial input and dip-aligned sandstone bodies, is typified by the upward progression: offshore mudstone--offshore transition (prodelta) mudstone to sandstone--d lta-front sandstone--estuarine and tidal distributary sandstone. In both profiles, subaerial beachface and shallowest marine lithofacies are rarely preserved due to erosion and replacement by estuarine sandstones during seaward strandline migration. Basinwide regression of the Point Lookout formation consists of numerous small-scale asymmetric transgressive-regressive cycles. Mappable erosive and/or non-depositional surfaces characterize transgressive events which separate genetically equivalent progradational sandstones. Duration of these small-order cycles is broadly estimated in the range of 104 years, probably at less than 50,000 years each. Cycle geometry is dependent upon the interplay of sedimentation rate and submergence, such that sedimentation rate controls the down-dip progradational sandstone width, and submergence influences the total cycle thickness. Because differences in thickness within the overall Point Lookout formation result from variability in cycle overlap (stacking), an understanding of cycle ge metry allows prediction of local stratigraphic pinchouts which may serve as stratigraphic traps for hydrocarbons. End_of_Article - Last_Page 541------------

Coal Stratigraphy of Deeper Part of Black Warrior Basin in Alabama: ABSTRACT

The Warrior field of Alabama is stratigraphically in the upper part of the Lower Pennsylvanian Pottsville Formation and structurally in the eastern part of the Black Warrior foreland basin. In the Warrior field, the Pottsville Formation has been divided into a lower non-coal-productive sandstone-rich sequence and an upper coal-productive sequence that contains less sandstone. The productive beds extend southwestward from the mining area downdip into the deeper part of the Black Warrior structural basin. Because the deep part of the basin is beyond the limits of conventional exploration, study of the stratigraphy of beds must rely on data from petroleum wells. For example, the density log as run in petroleum wells is sensitive to the presence of co l beds more than a few inches thick, but evidently does not provide a reliable measure of the thickness of individual beds. Relative abundance of can be stated in terms of numbers of beds, but because of the limitations of the available data, thicknesses of coals presently are not accurately determined. Distribution of beds in the Black Warrior basin reflects controls by depositional environments from delta plain to barrier islands; the various depositional environments shifted across the Black Warrior basin through time. The lower sandstone-rich coal-poor part of the Pottsville has been interpreted as barrier sediments in the mining area. To the southwest in the deeper Black Warrior basin, beds are more numerous within the sandstone-dominated sequence. The area of most numerous coals, trends northwestward parallel with the trend of the barrier islands, and evidently is in a back-barrier location. The coal-productive upper Pottsville is informally divided into coal (in ascending order, Black Creek, Mary Lee, Pratt, Cobb, Gwin, Utley, and Brookwood), each of which includes several beds. The Black Creek, Mary Lee, and Utley groups are associated with northeast-trending delta-distributary sandstones. The areas of most numerous coals also trend northeastward and are laterally adjacent to relatively thick distributary sandstones, suggesting accumulation in backswamp environments. The most numerous coals in the Pratt group are in an area that trends northwestward parallel with and southwest of a northwest-trending linear sandstone, suggesting accumulation in a back-barrier environment. Equivalents of the Cobb, Gwin, and Brookwood groups contain ittle in the deep part of the Black Warrior basin. End_of_Article - Last_Page 1473------------

Lower Wilcox Shelf Edge in Texas--Relation to Growth Faulting and Geothermal Reservoir Quality: ABSTRACT

Most geopressured sandstone reservoirs in the lower Wilcox (Eocene) of Texas occur along a narrow trend associated with the ancient shelf margin. Traps for geopressured fluids were created by early, rapid growth of down-to-the-basin faults as part of a large-scale instability of the continental slope. Basinward translation and rotation of upper-slope fault blocks over a decollement zone of geopressured shale (south Texas) or salt (east Texas) initiated fault movement near the shelf break. After the shelf edge had prograded farther basinward, continued movement of some of these faults at a much reduced rate created normally-pressured traps in post-lower Wilcox formations. In east Texas, the shelf-edge structural style has been overprinted by growth of salt dome. Faults tha originate near the restricts the volume of potential geothermal reservoirs. Lower Wilcox deposition was dominated by the Rockdale delta system in east Texas, similar in scale to delta system of the Quaternary Mississippi depocenter. Sand distribution reached its maximum extent when deltas prograded to the shelf edge. The thickest geopressured sands occur in De Witt County at the southern edge of the Rockdale system. Maximum permeabilities occur in distributary sandstones; these are laterally continuous with extensive delta-front sandstones of lower permeability. A proposed test-well site for the Cuero geothermal prospect in De Witt County has been located to intersect the greatest total thickness of distributary sandstones. End_of_Article - Last_Page 804------------

Anticipating Coal Mining Problems in Hartshorne Formation, East-Central Oklahoma, Using Sedimentary Facies Analysis: ABSTRACT

A study of sedimentary facies associated with the Hartshorne coal beds of east-central Oklahoma provides an opportunity for preventive planning to minimize mining problems. Sedimentary facies within the Hartshorne formation End_Page 724------------------------------ include delta distributary sandstones and interdistributary bar-fill shales, siltstones, and sandstones. One distributary channel sandstone body displays a shoestring geometry, over 12 mi (19.2 km) long and 1 mi (1.6 km) wide, with an average maximum thickness of 200 ft (61 m). This sandstone rests directly on and locally replaces the Lower Hartshorne coal bed. Three major mining problems are related to this sandstone body: (1) the Hartshorne sandstone in this area is a natural gas reservoir which might emit gas into adjacent coal mines; (2) the sandstone body is directly related to local discontinuities and rolls in the Lower Hartshorne coal bed; and (3) an unstable roof may be locally associated with trough cross-bedding and jointing near the base of the sandstone, and with facies changes and slickensides along the lateral margins of the sandstone body. In contrast to this sandstone body, interdistributary bay deposits, because of their relative homogeneity and lateral persistence, do not present potential facies-related mining problems. Potential mining problems associated with these facies are local and are directly related to structural and stress-release features which are difficult to predict in advance of mining. To insure safe and economic coal production from the Hartshorne formation, the distribution of major sandstone bodies overlying the coal beds must be considered when planning degasification programs and shaft, slope, and main entry locations. End_of_Article - Last_Page 725------------

Stratigraphy, Sedimentation, and Petrology of Oligo-Miocene Lower Frio Formation, Southwestern Louisiana: ABSTRACT

The Frio Formation, a major productive unit in South Louisiana, never has been described petrographically, although conventional cores and thousands of sidewall cores have been collected. The lower Frio consists of about 1,000 ft of alternating sandstone and shale in the updip stable shelf area, but it thickens to an observed maximum of 7,000 ft in the downdip unstable shelf (Nodosaria embayment) area. The lower Frio is mainly a regressive sequence, and has been divided into 4 units (designated A, B, C, and D from top to bottom) on the basis of electric-log correlations and other characteristics. Three lithologic sequences characterize each unit: a massive sandstone facies on the north (updip); an interbedded sandstone-shale (deltaic?) facies exhibiting considerable lateral variation; and a downdip sequence of alternating sandstone and shale in which the shale content increases markedly downdip. The sandstone bodies are thin and relatively persistent. Unit D, at the base, marks the beginning of the regression, has the least amount of sandstone, and has smallest amount of production. Shoreline and deltaic sandstone bodies are poorly developed. Unit C shows an increase in sandstone content and production. The deltaic facies is well developed, and shows a marked increase in volume and areal extent. Most of the production is associated with this facies, although production does occur in marine sandstone facies. Unit B shows only limited southward regression, but exhibits considerable development of marine facies. Numerous fields produce from this unit. Significant production also comes from channel-like bodies with erosional lower contacts. These bodies may be bar fingers or distributary channel deposits. Unit A is the most regressive. It contains the largest amount of massive continental and deltaic plain sandstones, as well as a well-developed delta system. As a result, marine sandstones are limited in number and extent. The distributary sandstone bodies account for major production and several giant fields. Independent stratigraphic, petrographic, and paleontologic studies demonstrate that the lower Frio contains strata deposited in continental, deltaic, and inner to middle neritic environments. Production, although primarily controlled by structure, is definitely affected by sedimentation and depositional patterns. Because structure in the Frio is now well known, future discoveries will be determined primarily by stratigraphic, petrographic, and paleontologic studies similar to this one. End_of_Article - Last_Page 643------------