Analysis of Oil Charging and Accumulation Processes in Tight Reservoir Beds: A Case Study of Lucaogu Formation in Jimsar Sag of Junggar Basin, NW China

As a hot and difficult research topic in the field of oil and gas geology, it is hard to clarify the tight oil charging and accumulating mechanism, migration and accumulation processes by a single approach. Taking the tight oil layers in the Lucaogu Formation in the Jimsar Sag of the Junggar Basin as an example, oil migration and accumulation processes in tight reservoir beds have been examined by combining physical modeling experiments of oil charging with the analysis of the process of diagenesis-reservoir formation-hydrocarbon generation and accumulation in this study. Has entered oil-generating window since Late Triassic period (corresponding to R_o=0.5%), and has entered massive hydrocarbon generation stage since Middle Jurassic period (corresponding to R_o=0.7%), the source rock in the Lucaogu Formation is still at the oil generation stage now. Many studies on hydrocarbon inclusions show that the Jimsar Sag has experienced two stages of hydrocarbon accumulation:the first stage was in Jurassic period (with inclusion homogenization temperature at 50~70℃, and the second stage was in Cretaceous-Paleogene period (with inclusion homogenization temperature at about 120℃). The diagenetic stages of tight reservoir beds in the Lucaogu Formation in the Jimsar Sag can be classified into three periods:①Period A in early diagenesis:early period of Late Permian (260 Ma) and before; ② Period B in early diagenesis:from early period of Late Permian to Late Triassic (260-220 Ma); ③Period A in middle diagenesis:from Late Triassic to present (220~0 Ma). According to analysis of diagenetic evolution stages and hydrocarbon accumulation process, and the conclusions from the physical modeling of oil charging in tight reservoir beds, the process of diagenesis-reservoir formation-hydrocarbon accumulation of tight oil in the Lucaogu Formation can be divided into the following three periods:(1) low-maturity oil and gas charging period under medium-high porosity and permeability condition in early reservoir beds (before Late Triassic); (2) the period that simultaneous tightness and accumulation occurs, and rock surface wettability changing to oilwet (from Late Triassic to the end of Early Cretaceous); (3) continuous charging period of mature oil in tight reservoir beds in the Lucaogu Formation (the end of Early Cretaceous to present). The Lucaogu Formation in the Jimsar Sag has large tight oil resource reserves potentials. The geological resources reserves of tight oil in upper sweet spot volume (Section B) are about 4.45×10⁸ t, and in lower sweet spot volume (Section E) are about 7.95×10⁸ t.