Origin and Enrichment Conditions of Shallow Shale Gas in the Qing 1 Member, Southeastern Uplift, Songliao Basin

ABSTRACT

Shallow shale gas has the potential to become an important alternative to conventional oil and gas because of its relatively low development cost, limited capital requirement, and rapid economic return. Although commercial success has been widely reported in medium‐ to high‐maturity shale reservoirs, breakthroughs in low‐maturity shale reservoirs remain limited. The organic‐rich shale of the first member of the Qingshankou Formation (Qing 1 Member) in the Wangfu Depression, Southeastern Uplift, Songliao Basin, is characterized by shallow burial and low thermal maturity. However, gas‐logging anomalies identified in several wells indicate considerable exploration potential. The results show that these shales followed a three‐stage relay gas‐generation model involving early biogenic gas, middle‐stage low‐maturity thermogenic gas, and late‐stage crude‐oil biodegradation gas. The present‐day shale gas is generally characterized predominantly by low‐maturity thermogenic gas, with signatures of biogenic gas and gases derived from crude‐oil biodegradation also present to some extent. Organic matter abundance (TOC and oil saturation index [OSI]) and maturity jointly control the capacity of the shale to generate thermogenic gas and subsequent biogenic gas. Shale gas mainly occurs in free and oil‐dissolved states and is hosted in intergranular pores, intercrystalline pores, and minor intragranular dissolution pores. During tectonic uplift, the adsorption capacity of the shale first decreases and then increases, whereas the free‐gas content continuously decreases. This differential evolution promotes the transformation of free gas into adsorbed gas, which favors the long‐term preservation of shale gas.