--> Factors Influencing Oil Heterogeneity in the Permain Loucaogou Lacustrine Tight Oil Reservoir in the Jimusar Sag, Junggar Basin, China

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Factors Influencing Oil Heterogeneity in the Permain Loucaogou Lacustrine Tight Oil Reservoir in the Jimusar Sag, Junggar Basin, China

Abstract

Abstract

The Permian Lucaogou Formation of Jimusaer sag in the southeast Junggar basin is the focus for tight oil exploration and development in the basin. Commercial oils have been obtained from many wells, and important advances in tight oil exploration have been achieved. The Lucaogou Formation contains fine-grained sediments dominated by mechanical and chemical sedimentation. The lithologies are composed by sandstone, mudstone, argillaceous siltstone, silty mudstone, dolomitic siltstone, dolomite mudstone, silt dolomite and argillaceous dolomite with characteristics such as fine particle size, thin single layers, and frequently interbedded lithology. Oil occurrence within the Lucaogou reservoir is strongly heterogeneous, even on the core sample scale. To determine the factors controlling oil content, new data from thin-section analysis, whole rock X-ray diffraction, total organic carbon, Rock-eval, Soxhlet extraction, porosity–permeability analysis, and mercury injection from more than 100 core samples are presented.

We determine the oil content using chloroform bitumen and pyrolysis parameter S1, which shows different characteristics due to lithologies. Oil content is high in some transitional lithologies such as argillaceous siltstone, silty mudstone, dolomitic siltstone, and silt-dolomite and low in crystal dolomite, fine-grained sandstone, and “pure” mudstone. The oil content is generally positively correlated with porosity, although in some samples, the oil content can be low even when the porosity is relatively high. The occurrence of high-porosity/low oil content reservoirs is explained by the rocks having a low oil generation potential and relatively small pore throat diameters, which means rich oil accumulation needs rocks to have high oil generation potential or large port throat diameters to allow oil to migrate in from outside. However, samples with a strong oil generation potential or suitable median pore radius with low porosity will always have low oil contents because of their poor storage capacity. The oil content in the Lucaogou tight reservoir is therefore controlled by a combination of porosity, pore structure, and hydrocarbon generation potential. Transitional lithologies, which have relatively favorable properties in respect of all three of these factors, consequently have high oil contents. These results provide important information for future exploration in the Lucaogou reservoir and for research on other lacustrine tight oil reservoirs.