DIAGENESIS AND RESERVOIR POTENTIAL OF PERMIAN-TRIASSIC FLUVIAL LACUSTRINE SANDSTONES IN THE SOUTHERN JUNGGAR BASIN, NORTHWESTERN CHINA/

The Junggar basin is one of the largest oil-producing areas in China, and contains Upper Permian lacustrine oil shales with some of the grea test hydrocarbon potental in the world, In this study, we present the diagenetic characteristics of Permian-Triassic sandstones from the sou thern Junggar basin and evaluate their reservoir potential. The upperm ost Permian and Lower Triassic Cangfanggou Group in the southern Jungg ar basin is characterized by alternating fluvial and lacustrine deposi ts, whereas the Middle-Upper Triassic Xiaoquangou Group was deposited predominantly in a lacustrine environment; fluvial and deltaic sedimen tation was subordinate. The sandstones of the Cangfanggou and Xiaoquan gou groups are volcanic litharenites. Their detrital modes and texture s of volcanic fragments suggest a primarily andesitic/basaltic volcani c-arc provenance. Early diagenesis of the sandstones is characterized by nonferroan calcite cementation, grain-coating, pore-lining clay min erals, and the initial dissolution of detrital grains. Authigenic quar tz; pore-filling phyllosilicates; pore-filling, grain-replacive zeolit es; albitized detrital plagioclase; authigenic K-feldspar; illite; and late calcite dominate burial diagenesis. The formation of iron oxides and dissolution of calcite cement resulted from tectonic uplift durin g the Tertiary. Albitization and zeolite formation during burial are a mong the most pronounced diagenetic processes that affected these sand stones. Pore-filling clay minerals, calcite, and zeolites have substan tially reduced sandstone porosity. However, appreciable primary porosi ty has been preserved by the formation of early clay coats and pore li nings, which retarded further cementation. Secondary porosity is prese nt to varying degrees in the sandstones and is the result of dissoluti on of unstable framework grains. In the Santai and adjacent areas of t he southern Junggar basin, the Cangfanggou Group sandstones can be mod erate to good quality reservoirs (measured core porosity ranges from 1 2.9 to 23.7%, averaging 18.2%), whereas the Xiaoquangou Group sandston es in general have relatively poor reservoir quality. Based on burial and thermal histories and diagenetic patterns, we predict that equival ent Permian-Triassic sandstone intervals in the enormous western part of the southern Junggar basin have potential as oil and (particularly) gas reservoirs.