Diagenesis of Non-marine Tight-gas Sandstones and Its Implications for Reservoir Quality Prediction: A Case Study onthe Lower Cretaceous Denglouku Formation, Songliao Basin, NE China

【摘要】：Diagenesis and reservoir-quality evaluation of non-marine tight-gas sandstones have been little studied. Lower Cretaceous Denglouku Formation sandstones in the Songliao Basin, northeastern China form the newly discovered tight-gas reservoirs. The sandstones, obtained from eight wells from three gasfields, mainly comprise moderate to well sorted, fine-to medium-grained feldspatic litharenites deposited in non-marine braided-fluvial to braid-delta settings under warm, arid to semi-arid climatic conditions. Data included description of cores, well logs, thin section petrography, and X-ray diffraction and scanning electron microscopy analyses. Diagenetic processes that have affected the Denglouku sandstones include compaction and precipitation of authigenic cements including carbonates(calcite, dolomite, and ferroan calcite), quartz, chlorite and illite. Porosity characteristics of the sandstones are closely linked to sediment textures and depositional facies, resulting in three groups of sandstones with distinctive diagenetic features:(1) coarser-grained sandstones deposited in braid-delta mouth bars and distributary channels and braided-fluvial channels; these sandstones contain relatively high porosity(up to 5%) with abundant chlorite and illite pore-lining cements but relatively minor carbonate cement;(2) coarser-grained sandstones deposited in braid-delta braid bars, distal bars and sand sheets and in braided fluvial braid bars, which contain little porosity due to a greater abundance of carbonate and quartz cements but low volumes of pore-lining clay minerals; and(3) finer-grained sandstones deposited in braid-delta mouth bars, distributary channels, braid bars, distal bars and sand sheets, and in braided-fluvial channels and braid bars; these sandstones have poor reservoir quality with the largest amounts of chlorite and illite pore-lining cements. A reservoir quality prediction model is proposed and incorporates textural, depositional and diagenetic attributes. The model will help to identify sweet spots in the Denglouku tight-gas sandstones and contribute to more effective field development. The results of this study can improve our understanding about the controls on reservoir quality, and may provide important input for exploration and development in non-marine tight-gas sandstone reservoirs in the Songliao Basin and analogous reservoir rocks elsewhere.