Robert S. Tye

Evolution of Atchafalaya lacustrine deltas, south-central Louisiana

Abstract The entrenched Pleistocene/Holocene Mississippi alluvial valley was cut during the last low sea-level stand, and alluviation of the entrenched valley occurred during the Holocene sea-level rise. Regional Gulf Coast structures (salt domes and faults) have strongly influenced the morphology of the valley. Valley paleotopography has controlled depositional patterns and locations of major river courses, as well as partially isolated large interdistributary basins within the alluvial/deltaic plain. Interdistributary basin formation is further enhanced by distributary channel progradation, switching, and abandonment, in conjunction with compactional subsidence of the alluvial plain surface. Geomorphology and sediment cores from the Atchafalaya Basin, Louisiana, revealed that this basin has alternately accumulated fine-grained organic-rich sediments and terrigenous clastic-dominated meander-belt and lacustrine deposits. Basin subsidence and the presence of an active distributary channel in the basin are major determinants of the lithology of the sedimentary-fill. Prograding lacustrine deltas of the Atchafalaya River are the primary mechanism for filling the Atchafalaya Basin. These upward-coarsening deltas are relatively thin (3 m) but are areally extensive (tens to hundreds of kilometers), and because they occur so frequently during the infilling process, they account for a significant volume of the basin sequence. The Atchafalaya Basin sequence documents lacustrine deltaic episodes dating from the early Holocene (Maringouin and Teche deltas) to the present (Atchafalaya delta). Continued development of the Atchafalaya River will rework portions of the backswamp and lacustrine deposits into meander-belt deposits. Upon complete filling of the Atchafalaya Basin, sediment will mostly bypass the basin and become deposited in the marine delta in Atchafalaya Bay.

Wetland loss in Louisiana

ABSTRACTCoastal wetland loss in Louisiana, now considered to amount to more than 100 km2/year, is receiving ever increasing amounts of attention. This loss is the result of a variety of complex int...

Geomorphology: An approach to determining subsurface reservoir dimensions

Modern depositional settings provide unambiguous geomorphic data facilitating the quantification of geologic interpretations for the numerical characterization of subsurface strata. Traditional three-dimensional geologic descriptions are limited by one- and two-dimensional data sources: generally well and outcrop data. However, geomorphic analyses of fluviodeltaic systems yield size distributions for discrete sedimentary units. These distributions provide constraints for conditioning the area (X and Y dimensions), shape, placement, and preferred orientation(s) of sedimentary units in reservoir models.Dimensional data for channel belts, channels, channel bars, crevasse splays, distributary channels, and distributary mouth bars from the fluvially dominated deltas of the Alaska North Slope and the Louisiana Gulf Coast reveal log-normal distributions for their lengths and widths. Modern analogs provide statistical constraints for conditioning data input for geologic facies associations in object-based reservoir models.Geomorphic data from modern fluviodeltaic analogs are linked with core and wire-line–log data to render conditioned, three-dimensional geologic models. Model accuracy relies on bed thickness and chronostratigraphic constraints imposed by cores and stratigraphic correlations, as well as the lateral extent of facies associations governed by geomorphology. These object-based reservoir models demonstrate the impact that varying the population of lengths and widths for geologic features has on sand body distribution and interwell continuity.

Origin of Crude Oil in the Wilcox Trend of Louisiana and Mississippi: Evidence of Long-Range Migration

ABSTRACT Geochemical characterization of crude oils from Wilcox reservoirs in central Louisiana and southwest Mississippi suggests that they represent a single crude oil family that is distinct when compared to crude oils in deeper Tuscaloosa and Smackover reservoirs. This observation is consistent with geologic constraints that suggest an origin of crude oil from within the Wilcox Group itself. Although shales of the shallow Wilcox Group in central Louisiana and southwest Mississippi contain gas-prone kerogen and are thermally immature, a more oil-prone source facies is present in marine shales of the deep Wilcox Group in southcentral Louisiana. Thermal maturity measurements based on pyrolysis suggest a broad area of effective Wilcox source rock in southcentral Louisiana. Migration distances from source to reservoir rocks of the downdip Wilcox Trend of southcentral Louisiana appear to be relatively short. However, long-range updip migration (sometimes greater than 100 km) from deeply buried Wilcox source facies provides the best explanation for emplacement of crude oil in the shallow Wilcox Trend of central Louisiana and southwest Mississippi.

ABSTRACT: Unique Horizontal-Well Designs Boost Primary and EOR Production, Prudhoe Bay Field, Alaska

U sing horizontal wells and an unprecedented enhanced oil-recovery (EOR) process, ARCO Alaska, Inc., capitalized on structural, stratigraphic, sedimentologic, and fluid-distribution complexities in the Triassic Ivishak Sandstone of Prudhoe Bay field to recover untapped or bypassed reserves. Millions of barrels of reserves reside in a basal, 120 ft(37 m-) thick succession of en echelon, offlapping, deltaic wedges. Distributary-mouth bar and distributary-channel sandstones are targeted. Onlapping, retrogradational mudstones form laterally extensive flow barriers that impede the gravity-drainage