The enrichment conditions and model of shale gas reservoir in the Chang 7 member of Mesozoic Yanchang Formation in Yan'an, Ordos Basin

This paper is the result of oil and gas exploration engineering.

Objective The Chang 7 shale of Mesozoic Yanchang Formation in Ordos Basin is characterized by rapid changes of lithologies and lithofacies, thick, high abundance but low maturity of organic matter, high clay content and low-normal strata pressure. The study on shale gas enrichment conditions and development model of the Chang 7 shale is beneficial to explore the geological theory suitable for the exploration and development of Chang 7 shale gas, in order to provide guidance for the next exploration work in the region and further improve the geological theory of continental shale gas.

Methods Based on drilling, core analysis, and test data, the accumulation and enrichment conditions of the Chang 7 shale gas were systematically studied from the aspects of hydrocarbon generation conditions, reservoir conditions and preservation conditions, and the enrichment and development model of shale gas was established.

Results The results show that: (1) Chang 7 shale in the study area has high organic matter abundance, TOC is basically greater than 1.5%, and is in the oil and gas generation window, Ro is basically between 0.8% and 1.3%, and has the geological conditions of shale gas generation; (2) The clay mineral content is high, basically 40%-60%, which is beneficial to shale gas adsorption and has a certain adsorbed gas production; (3) The relatively stable tectonic background is an important condition for the preservation of shale gas; (4) The enrichment model of shale gas can be divided into two types: interbedded shale and sandy lamina type, and thick organic-rich shale type. The sandy lamina effectively improves the physical properties of shale reservoirs. The thick organic-rich shale deposited continuously is an important form of gas preservation.

Results Based on the dissection of the enrichment characteristics of Chang 7 shale gas, the enrichment law of Chang 7 shale gas is summarized as "two large and one many": large gas generation potential, large continuous thickness and many sandy laminas.