DIAGENESIS OF TIGHT-GAS SANDSTONES IN THE LOWER CRETACEOUS DENGLOUKU FORMATION, SONGLIAO BASIN, NE CHINA: IMPLICATIONS FOR RESERVOIR QUALITY

摘要

Non-marine tight-gas sandstones have not been studied intensively in terms of diagenesis and reservoir-quality evaluation. This paper investigates the sandstones in the Lower Cretaceous Denglouku Formation in the Songliao Basin, NE China, which comprise a tight reservoir for natural gas. Core samples of the sandstones were obtained from eight wells located in two gasfields and were described in terms of lithology and sedimentology with the aid of wireline log interpretation; other data included thin section petrography, and X-ray diffraction analyses and scanning electron microscopy. The sandstones mainly comprise moderate- to well-sorted, fine- to medium-grained feldspathic litharenites deposited in braided fluvial to braid-delta settings under warm, arid to semi-arid climatic conditions. Diagenetic processes which have affected the sandstones include compaction and cementation by calcite, dolomite, ferroan calcite, quartz, chlorite and illite. Porosity characteristics of the sandstones are closely linked to depositional facies, and the sandstones can be divided on the basis of sediment textures and depositional facies into three groups which have distinctive diagenetic features. The best reservoirs are represented by relatively coarse-grained sandstones deposited in braid-delta mouth bars and distributary channels and braided fluvial channels; these sandstones have relatively high porosity (up to 5%) and contain large amounts of chlorite and illite pore-lining cements but relatively minor carbonate cement. A reservoir quality prediction model is proposed and incorporates textural, depositional and diagenetic attributes. The model will help to identify sweet spots in non-marine tight-gas sandstones elsewhere, and will contribute to more effective field development.