RESERVOIR POTENTIAL OF DEEP-WATER LACUSTRINE DELTA-FRONT SANDSTONES IN THE UPPER TRIASSIC YANCHANG FORMATION, WESTERN ORDOS BASIN, CHINA

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; laterallyextensive 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.