Source-rock reservoirs: Ring structure characteristic, differential accumulation mechanism and ordered distribution pattern

doi:10.13745/j.esf.sf.2022.8.26

Abstract

Abstract:

The future direction of oil and gas exploration is to find oil near or inside the source-rock area. However, this type of operation is still risky due to lack of understanding of source-rock properties, mainly its hydrocarbon accumulation conditions, mechanisms and distribution laws. This paper redefines the ‘source-rock system’ and ‘source-rock reservoir’ concepts—it considers the source-rock system is an independent hydrocarbon accumulation unit, and source-rock reservoir is hydrocarbon accumulation within the source-rock system. Hydrocarbon reservoirs are divided into two categories: conventional and unconventional reservoirs. There are six types of hydrocarbon source-rock reservoirs, including mudstone fractured reservoir, lithologic reservoir, shale oil and gas reservoir, and tight oil and gas reservoir. Under structural control, each reservoir type has its unique formation mechanism and specific distribution law and together they form a ‘horizontal parallel ring’ structure, as the source-rock system—with vertical sequence formations, upper and lower stratification transition, differential formation mechanism (conventional vs. unconventional), and ordered symbiosis—developed from deep lacustrine shale and turbidite sandstone reservoirs in the inner circle, to semi-deep lacustrine tight reservoirs in the middle circle, then to shallow lacustrine and semi-deep lacustrine transition facies in the outer circle. The source-rock system is formed by orderly, sequential accumulation mechanism, where the shale reservoirs formed in the inner ring via in situ continuing accumulation and micro migration interruption; the tight reservoirs formed in the middle ring via large-scale primary migration interruption; and the lithologic pinch-out reservoirs formed in the outer ring via buoyancy secondary migration closure. This paper determines that the shale reservoir “sweet spots” are located in the predelta facies belt of deep basin inner circle with “dominant fabric and superior hydrocarbon retention”; for tight oil and gas reservoirs the “sweet spots” are located in the “dominant pore-fracture coupling” region of delta front facies belt in the middle circle of deep basin; and for lithologic pinch-out reservoirs the favorable distribution areas are located in the trap area with dominant migration channel along convergent ridge in delta plain facies belt outside the deep basin. There are variety of source-rock reservoirs with huge oil and gas resources potentials, and their exploration, development and utilization has just begun.

Key words: source rock reservoir, circle effect, accumulation mechanism, enrichment mode, the orderly distribution

CLC Number:

P618.130.2

LUO Qun, ZHAO Xianzheng, PU Xiugang, JIN Fengming, JIANG Wenya, ZHANG Hongli, QIU Zhaoxuan, WEN Pan. Source-rock reservoirs: Ring structure characteristic, differential accumulation mechanism and ordered distribution pattern[J]. Earth Science Frontiers, 2023, 30(1): 174-186.

Figures/Tables 4

Fig.1 Ring structure,differential formation mechanism and ordered distribution pattern of source rock reservoirs in oil-rich sag and deep basin area

Fig.2 Microscopic source-reservoir coupling and retention oil-rich mechanism model of shale with obvious lamellar differentiation in the Second Member of Kongdian Formation in Cangdong Sag

Fig.3 Ring structure and hydrocarbon reservoir formation mechanism and occurrence state of source rock in deep basin area of basin or sag. Modified after [42].

Fig.4 Comprehensive map of ring structure,sedimentation and oil and gas reservoir distribution of the second Member of Kongdian Formation in Cangdong Sag. Modified after [42].

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