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Abstract The structural and stratigraphic framework of Maraven's Block I was re-interpreted using 3-D seismic and existing data as part of an evaluation of the remaining oil potential. More than 1800 MMBO have been produced from Block I in the past 40 years, mainly from structural traps. In order to maintain production levels, it has become increasingly important to define the seismic stratigraphic framework for the area and to accurately locate faults and stratigraphic pinchouts. The dominant structures are the Icotea fault, its conjugate fault system, and the Eastern Boundary fault. The most prominent fault is the NE-striking Icotea fault, which subdivides the area into two main structural blocks, a graben in the West Flank and a horst in the East Flank. The Icotea fault is a highly complex fault zone with a long history of deformation. It is a nearly continuous fault zone with both vertical and lateral offsets and is locally inverted. Along the eastern flank of the Icotea, prominent reverse-fault bounded upthrown blocks, called the Attic, have developed. Along the western flank, contraction has re-activated listric faults into reverse and thrust faults. Major northwest-striking normal faults delineate a large paleoarch that occurs in the south-center of the East Flank. This phase of faulting produced small horst and graben blocks bounded by normal faults that dip to the northeast and southwest. The Eastern Boundary fault is subparallel to the Icotea fault and is an east-dipping normal fault that has been locally inverted and occurs in a synclinal area of the block. Two play concepts, utilizing (1) horizontal wells in Attic and (2) vertical wells along the Eastern Boundary fault, were successfully tested during this study. The stratigraphic section includes, from oldest to youngest, pre-Triassic basement rocks; the Jurassic graben-fill Quinta Formation; the Cretaceous Rio Negro, Cogollo Group, La Luna, Colon, and Mito Juan formations; the Paleocene Guasare Formation; the Eocene Misoa Formation; the Miocene La Rosa, Lagunillas, and La Puerta formations; and the Quaternary El Milagro Formation. Only the lower part of the Eocene Misoa Formation (C sands) is preserved in Block I, and most of the Eocene B sands and all of the Pauji were either eroded or not deposited in this area. The main reservoirs occur in the Eocene Misoa Formation and the basal Miocene Santa Barbara member of the Lagunillas Formation. Sedimentation occurred throughout the Eocene and was strongly influenced by tectonism. The Eocene section in the horst block is up to 760 m thick and is bracketed by two major unconformities. The upper angular unconformity places the basal Miocene Santa Barbara member (16–25 Ma) over the Eocene Misoa C sands (45–54 Ma). The lower disconformity (54 Ma) occurs at the top of the Paleocene Guasare Formation. In between, eight seismic sequences occur within the Eocene horst section. The adjacent stratigraphic sections east and west of the horst block are thicker than the East Flank section. The C sands in Block I form a retrogradational clastic sequence deposited as transgressive (70–80%), highstand (10–15%), and lowstand wedge and incised valley fill (10–15%) systems tracts with prominent marine-flooding surfaces separating these systems tracts. The main reservoirs are thick-bedded transgres-sive sandstone deposits.

ABSTRACT The depositional environments and reservoir characteristics of the upper Miocene Etchegoin and Chanac formations in Kern Front oil field, eastern San Joaquin basin, California, were interpreted in cores from three wells. Sedimentary facies were correlated to the log responses and then mapped throughout the reservoirs. The Etchegoin Formation is a shallow-marine unit consisting of a basal transgressive and overlying deltaic units. Facies include shoreface, river-mouth bar, prodelta, paralic, and marine and distributary channel deposits. The shoreface deposits trend north-south, and are interbedded with, and overlain by, bioturbated marine units. They are gradational upward into river-mouth bars and marine channels, and are incised locally by distributary channels. The Chanac Formation underlies the Etchegoin Formation and contains meandering stream sequences deposited on a low-relief, mud-rich coastal plain. West-trending channels are recognized in the Chanac Formation. Log-derived data, combined with core porosity and permeability measurements, indicate that the upper Miocene reservoir sandstones have an average porosity 36.5% and average permeability 2,200 md. The Etchegoin sandstones are poorly indurated, arkosic arenites and wackes composed of subangular, medium- to coarse-grained, poorly-sorted detritus. Detrital modes for the sandstones are Q33 F47 L20 and Qm38 P48 K14. Authigenic minerals are rare and include calcite, illite, and Ca-zeolite. The best reservoir sandstones in the field are the Etchegoin deltaic shoreface and channel facies and the Chanac channel facies. The two factors controlling reservoir quality are grain size and sorting. The proportion of detrital matrix ranges from 11 to 12% in the better reservoir facies to 22 to 45% in the poorer reservoirs; the proportion of the coarsest-grained fraction (sand/gravel) ranges from 54% in the poorer reservoirs to 83% in the better reservoir facies.

ABSTRACT Thirty-five turbidite sand bodies from the Moco T and Webster reservoir zones were delineated for EOR projects in Mobil’s MOCO FEE property, south Midway-Sunset Field. The recognition of these sand bodies is based on: mappable geometries determined from wireline log correlations, log character, core facies, reservoir characteristics, and comparison to nearby age-equivalent outcrops. These turbidite sands are composed of unconsolidated arkoses of late Miocene age (“Stevens equivalent,” Monterey Formation). The sand bodies are dip oriented and parallel the northeast-dipping paleoslope. Reservoir quality in the sandstone is very good with average porosities of 33%, permeabilities of 800 to 4,000 md, and average oil saturation of 32 to 65%. The underlying Moco T Zone has 20 mappable sand bodies. Their mapped dimensions average 40 feet thick, 1,600 feet wide, and 5,000 feet long. These deposits are coarse grained, poorly sorted, amalgamated, thin and fine upwards, and contain shale intraclasts, dish structures, and slumped intervals. The sand bodies are lenticular, shale out to the east and west, and are interpreted to be channel-fill deposits. The overlying Webster Zone is divided into two units and 15 sand bodies. The Webster sands are similar in dimensions to those of the Moco T, but document a change in sedimentary character from medium-grained, sheet-like depositional lobes in the lower Webster Main to coarse-grained, lenticular channel-fill deposits in the upper Webster Intermediate. A channel/lobe transition zone occurs between the two facies. The depositional lobes average 28 feet thick, 6,000 feet wide, and 5,600 feet long; channel-fill sand bodies average 37 feet thick, 1,100 feet wide, and 4,000 feet long; and channel/lobe transition deposits average 33 feet thick, 3,200 feet wide, and 4,200 feet long. The depositional lobes contain sandstone beds with Bouma sequences and mudstone interbeds that are locally bioturbated, whereas the channel-fill units are poorly sorted, amalgamated, and contain {boulders and shale intraclasts.

Abstract Walker Lake sedimentary basin is a fault-controlled continental basin related to strike-slip faulting on the western side of the Basin and Range Province of Nevada. The Walker Lake Basin is contained within a triangular crustal block bounded by normal-to oblique-slip faults on the west, left-lateral faults on the south, and right-lateral strike-slip faults on the east (Walker Lane shear zone). Modern Walker Lake is roughly one fourth the surface area and the water depth of its Pleistocene precursor Carbon-rich (up to 2.5% total organic carbon) and uranium-rich sediments are currently accumulating in the deeper saline and anoxic parts of Walker Lake. If these conditions were to continue, significant potential hydrocarbon source rocks and uranium-bearing beds could accumulate. Walker Lake Basin is being infilled by axially fed, sand-rich fluvial-deltaic deposits; side-fed, coarse-grained alluvial-fan/fan-delta deposits; and central fine-grained lacustrine deposits. Waves, wind, and lake-level fluctuations have caused reworking of the lower parts of fan-delta surfaces and the front (windward side) of the Walker River delta. Carbonate deposits, which include beach-rock horizons, stromatolites, oncolites, caliche, and tufas, locally form along the shorelines and spring areas of this predominantly coarsegrained clastic system.