A Quantitative Model of Secondary Pore Evolution for Tight Sandstone Reservoirs and the History of Hydrocarbon Charging: Yingcheng Formation, Lishu Fault Depression, China

Abstract

During the hydrocarbon charging period, reservoir pore size controls the formation mechanism and distribution law of a reservoir. In this work, we aimed to develop a porosity quantitative restoration model for tight sandstone reservoirs and reconstruct the historical process of hydrocarbon accumulation. The research methods employed were core description, X-ray diffraction, scanning electron microscopy, fluid inclusion, basin modeling, and stable carbon and oxygen isotope analysis. The findings revealed that the reservoir spaces in sandstones of the Yingcheng Formation comprise dissolution pores, microfractures and micropores, with the majority of core samples exhibiting average porosities and permeabilities of 3.6% and 0.7 mD (1 mD (millidarcy) = 9.869233 × 10⁻¹⁶ m²), respectively. The reservoir has experienced four main diagenetic effects, namely, early compaction, early cementation, middle dissolution and late cementation, and is currently in the mesodiagenesis B to telodiagenesis stage. Basin modeling revealed that the source rocks of the Shahezi Formation reached the hydrocarbon generation threshold at 107 Ma and reached the overmature stage at 89 Ma. The porosity evolution analysis revealed that the primary sedimentary porosity (\({\Phi }_{0}\)) is 36.6%. At the end of eodiagenesis A (\({\Phi }_{\text{ea}}\)), the porosity stood at 12.2%; at the end of eodiagenesis B (\({\Phi }_{\text{eb}}\)), it declined to 6.9%; following mesodiagenesis A (\({\Phi }_{\text{ma}}\)), it reached 9.1 %; and after mesodiagenesis B – telodiagenesis (\({\Phi }_{\text{mt}}\)), it was recorded at 4.8%. The history of natural gas charging indicated that the main charging period for natural gas was approximately 98.5–94.5 Ma. Therefore, the natural gas reservoirs of the Yingcheng Formation are classified as “hydrocarbon accumulation after sandstone densification”. The findings elucidate the accumulation process of tight sandstone gas and offer insights for applying these methods in other regions.