Diagenesis and reservoir quality of deep-lacustrine sandy-debris-flow tight sandstones in Upper Triassic Yanchang Formation, Ordos Basin, China: Implications for reservoir heterogeneity and hydrocarbon accumulation

Abstract

Tight sandstone in the Upper Triassic Yanchang Formation, Ordos Basin, contains large accumulations of hydrocarbon resources. This study presents an investigation of the deep-lacustrine sandy-debris-flow tight sandstones in the Yanchang Formation, based on petrographic, geochemical and petrophysical analysis. It reveals that reservoir heterogeneity and hydrocarbon accumulation of tight sandstones were affected by diagenetic alteration. These tight sandstones are dominantly lithic arkoses and feldspathic litharenites which are characterized by moderate to good sorting, fine to medium grain, subangular to subrounded and poor reservoir properties (average porosity and permeability are 10.5% and 1.23 mD, respectively). Significant diagenesis including compaction, feldspar dissolution and cementation by carbonate, quartz and authigenic clay have altered the studied tight sandstones. Compaction and carbonate cementation are the dominant destructive diagenetic processes. Carbonate cements are mainly developed at the edge of sandstone (distance to the sandstone-mudstone boundary mostly < 1 m) due to the limited transportation distance from adjacent mudstone. The relative high-quality reservoir intervals will preferentially develop in the middle of sandstone (distance to the sandstone-mudstone boundary mostly > 1 m). Feldspar dissolution has no significant impact on reservoir quality, as it results in pore space redistribution. There is little variation due to the primary intergranular pores being converted to dissolution pores and intercrystal micropores of authigenic clay. The lower level of compaction in the middle sandstone is due to cemented marginal sandstone. Moreover the preservation and redistribution of the pore space of the middle sandstones is generally due to carbonate cementation and feldspar dissolution. Different petrophysical properties between marginal and central sandstones lead to reservoir heterogeneity. Reservoir petrophysical properties display a positive correlation with oil-bearing capacity, which reveals that hydrocarbon accumulation can also be affected by reservoir property. Ultimately, tight sandstones with porosity >6%, permeability >0.1mD and thickness >2 m can be regarded as effective reservoirs in the Yanchang Formation. The results will contribute to the understanding of the origin of deep-lacustrine sandy-debris-flow tight sandstones reservoirs, which will enable further exploration for effective reservoirs in lacustrine sandy debrites across the globe, predominantly in those sharing the same geological features.