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The Baxian depression is a typical half-graben located in the Jizhong sub-basin, north China. Commercial petroleum traps have been discovered in the Jizhong sub-basin. However, the 3rd and 4th members of the Shahejie Formation in this sub-basin have been poorly explored. These two members, belonging to the Lower Paleogene age, are buried deeply in the depression. Favorable petroleum reservoir conditions exist in such deep intervals of the half-graben due to the presence of different types and extent of deltas and turbidity fans in various areas. In fact, three types of turbidite fans are developed in the sag below the transitional belt on the eastern gentle slope. This work summarized three stratigraphic trap belts, i.e., the steep slope, gentle slope, and sag. On the steep slope, structural-stratigraphic traps with small-scale delta fronts and turbidite sandbodies are well developed. On the gentle slope, hydrocarbons generally accumulate in the large-scale delta front, onlapping beds and those sandbodies adjacent to unconformities. In the sag, petroleum trap models are typically characterized by pinched-out turbidite sandbodies. Stratigraphic traps were easily formed in turbidite fans below the eastern transitional belt. The petroleum traps that have already been discovered or predicted in the study area indicate that stratigraphic traps have favorable petroleum exploration potential in deeply buried areas (depth >5000 m) in a half-graben basin or depression. [ABSTRACT FROM AUTHOR]

This paper investigates the reservoir characteristics of deep-water lacustrine-delta sandstones in the Upper Triassic Yanchang Formation in the western Ordos Basin, north-central China. The Yanchang Formation has previously been interpreted as a shallow-water lacustrine deltaic succession, and this interpretation has been used to guide petroleum exploration activities which have however met with only limited success. The present study integrates thin-section, wireline log, X-ray diffraction and SEM data from wells in the western Ordos Basin to determine the sedimentary and diagenetic characteristics of sandstones in the C6 and C4+5 sub-members of the Yanchang Formation, and to interpret the units' depositional environment. The C6 and C4+5 sub-members in the study area are composed of: mudstones and fine-grained sandstones, which are interpreted as deep-water pro-delta deposits; laterally-extensive sand sheets (outer delta-front deposits); and small-scale distributary channel and mouth bar sandbodies (inner delta-front deposits). The sandstones have reservoir potential but diagenesis has had a range of effects on reservoir quality. Compaction together with cementation by calcite and clay minerals including chlorite and kaolinite may have affected pore throat geometry and permeability adversely. However dissolution of feldspars and calcite cement created secondary porosity. Hydrocarbon accumulations may occur in delta-front channel and mouth bar sandbodies. Core studies show that some sandstone intervals have relatively good reservoir properties with porosity up to 15% and permeability up to 9mD. [ABSTRACT FROM AUTHOR]