Lithofacies Prediction and 3-D Geological Model in Tight Gas Sandstone Reservoirs by Integration of Well Logs and Geostatistics Modeling

Abstract

Understanding reservoir heterogeneity is important for the economic success of tight gas sandstone reservoirs. The intrinsic complex of a reservoir is result of depositional environment and digenesis. In the Upper Paleozoic Shanxi Formation of the Ordos Basin, primary sandstone texture and composition (grain size, ductile lithic sand grains) directly related to depositional variability in the delta-front environments, control compaction, the abundance of cements and porosity.

This study highlights the upscaling of small-scale (pore- and core-scale) lithofacies heterogeneity for accurate filed-scale geological modeling using well logs. Five lithofacies are defined based on detrital texture and composition, diagenetic features, and pore space properties. Detrital quartz-rich sandstones (quartzarenitic and rare sublitharenitic) show the highest reservoir quality; these sandstone can be identified based low gamma ray, low bulk density, and high deep resistivity log values. Sandstones of the poorest reservoir quality are rich in ductile lithic grains and clay matrix or tightly cemented with carbonate. Ductile lithic-rich sandstones (litharenitic and most sublitharenitic) are identified by high gamma ray, high bulk density, and low deep resistivity value. Tight carbonate-cemented intervals are identified by high bulk density value. Tuffaceous quartz sandstones with poor reservoir quality show some overlaps in well logs from other lithofacies. A model based on principal component analysis (PCA) show better identification of the five lithofacies than biplots of well logs.

A 3D architectural model of channel-belt is constructed based on multiple evolutionary maps of regional sedimentary microfacies. Lithofacies are modeled further by interpreted well data using sequential Gaussian method. At field scale, detrital quartz-rich sandstones are distributed mainly in the lowermost and uppermost intervals of the Shanxi Formation. Tuffaceous quartz sandstones are found only in the lowermost intervals. At a channel scale, the dominant lithofacies are detrital quartz-rich sandstones in the middle–lower interval of the distributary channels, changing gradually upward into ductile lithic-rich sandstones. The 3D lithofacies model is validated by correlation with gas production test which suggests it is helpful predictive model for sweet spot.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018