Fluid-rock interactions and porosity genesis in deep clastic reservoirs: A perspective of differential oil charge intensity

Abstract

Fluid-rock interactions are important factors affecting pore genesis and hydrocarbon accumulation in sedimentary basins. Different responses and processes will occur under differential oil or gas charge intensities, which is important for reservoir evaluation and prediction, but few studies focus on this topic. To fill this knowledge gap, petrologic, major element, and isotopic study were conducted in conglomeratic reservoirs of the late Permian Upper Wuerhe Formation (P3w) in the Shawan Sag, Junggar Basin, northwestern China. The results show that tuff and volcanic debris are enriched in the formation, and laumontites generally occur as cements due to the alteration of volcanic materials. In oil and gas reservoirs undergoing strong hydrocarbon charge, laumontite and calcite cements generally exist, and partial of cements, as well as some debris and feldspar, are dissolved, forming abundant secondary porosity. The two cements also exist in the water-bearing and dry layers, but both of them are only pratially dissolved. This differential dissolution implies the reformation of acidic hydrocarbon-bearing fluid in clastic reservoirs. Mineral assemblages show that laumontite and small amounts of early-stage calcite precipitated during eodiagenesis. During the continuous compaction of the reservoir, first-stage hydrocarbon charge occurred in the Middle Jurassic when primary porosity almost disappeared. Hydrocarbon charge inhibited the precipitation of laumontite and calcite, promoting their dissolution. Second-stage hydrocarbon charge in the Early Cretaceous further caused more extensive dissolution of laumontite and calcite, forming abundant secondary pores. Authigenic quartz, kaolinite and late-stage calcite precipitated as the associated minerals of laumontite dissolution. Late-stage calcite is characterized by Mn-rich (>2.5%) and δ13C depleted deviations (−19.5‰ ∼ −14.3‰, VPDB), recording the strong activities of hydrocarbon-bearing fluids. This study emphasizes the importance of reservoir reconstruction after hydrocarbon charge and suggests favourable hydrocarbon accumulations may be found in deeply buried clastic reservoirs.