Unconventional natural gas accumulation system

Abstract

Unlike conventional natural gas accumulations, unconventional ones have obvious particularity in reservoir forming conditions, enrichment mechanism, distribution modes, prediction method, and exploration technology. Analyzing the reservoir forming process and main controlling factors of unconventional natural gas accumulations under the guide of the hydrocarbon accumulation system theory is of great significance for unconventional natural gas reservoir evaluation and prediction. Based on the reservoir forming mechanism and distribution mode, the unconventional natural gas accumulation systems are divided into six types, including shale gas, coalbed methane, tight carbonate gas, tight sandstone gas, water-soluble gas, and gas hydrate. According to the concept of “element–function–structure” in the hydrocarbon accumulation system theory and the relationship of “gas source rock–migration system–accumulations distribution”, the unconventional gas accumulation system is divided into six “source–location” types. The characteristics of the other five types of unconventional gas accumulation systems except gas hydrate are analyzed. The following results were obtained. (1) Shale gas, coalbed methane and tight carbonate gas belong to an intra-source type unconventional gas accumulation system, characterized by intense gas supply capacity, lack of either a transport system or secondary migration, integration of source, reservoir and caprock, in-situ accumulation, and so on. (2) Tight sandstone gas and water-soluble gas belong to a marginal unconventional gas accumulation system of source rock, characterized by diverse gas origin, near-source accumulation in adjacent layers of source rocks, internal migration within reservoirs, and so on. (3) Basin tectonic evolution, sedimentary environment and late tectonic events control the types of unconventional gas accumulation systems: The Paleozoic marine sediments in South China are conducive to the formation of shale gas and tight carbonate gas accumulation systems; The marine–continental transitional facies deposits of Upper Paleozoic distributed in the South and North China are conducive to the development of coalbed methane, shale gas and tight sandstone gas accumulation systems; In the Middle and East China, Mesozoic–Cenozoic deep-water continental deposits are more conducive to the deep tight sandstone gas and shale gas accumulation systems, while shallow water continental basins are more conducive to the formation of coalbed methane, tight sandstone gas and shale gas accumulation systems. Water-soluble gas is widely found in the Cenozoic shallow-buried areas rich in organic matter or high-pressure reservoirs adjacent to gas source rocks.