Abstract
To clarify the characteristics and enrichment rules of Paleogene tight sandstone reservoirs inside the rifted-basin of Eastern China, the third member of Shahejie Formation (abbreviated as Es3) in Wendong area of Dongpu Depression is selected as the research object. It not only clarified the geochemical characteristics of oil and natural gas in the Es3 of Wendong area through testing and analysis of crude oil biomarkers, natural gas components and carbon isotopes, etc.; but also compared and explained the types and geneses of oil and gas reservoirs in slope zone and sub-sag zone by matching relationship between the porosity evolution of tight reservoirs and the charging process of hydrocarbons. Significant differences have been found between the properties and the enrichment rules of hydrocarbon reservoirs in different structural areas in Wendong area. The study shows that the Paleogene hydrocarbon resources are quasi-continuous distribution in Wendong area. The late kerogen pyrolysis gas, light crude oil, medium crude oil, oil-cracked gas and the early kerogen pyrolysis gas are distributed in a semicircle successively, from the center of sub-sag zone to the uplift belt, that is the result of two discontinuous hydrocarbon charging. Among them, the slope zone is dominated by early conventional filling of oil-gas mixture (at the late deposition period of Dongying Formation, about 31–27 Ma ago), while the reservoirs are gradually densified in the late stage without large-scale hydrocarbon charging (since the deposition stage of Minghuazhen Formation, about 6–0 Ma). In contrast, the sub-sag zone is lack of oil reservoirs, but a lot of late kerogen pyrolysis gas reservoirs are enriched, and the reservoir densification and hydrocarbon filling occur in both early and late stages.
Highlights
The efficient exploration and development of tight oil and gas resources in the 21st century has transformed the structure of the global energy (Zou et al, 2012; Wang et al, 2016)
It is not difficult to see that type A crude oil has low maturity and is mainly distributed in Es2 and the upper subsection of Es3 in slope area, which is supplied by mixed source rocks with multiple sets of dark mudstones inside Es2 and Es3 members
According to the genesis of natural gas, the reservoir densification process and the matching relationship between reservoir compaction and hydrocarbon accumulation, the deep Paleogene oil-gas reservoirs in Dongpu Depression can be divided into three types: (1) a “reservoir-forming densification” type of early oil reservoirs and early kerogen cracking gas reservoirs, (2) a “simultaneous densification and reservoir-forming” type of latestage oil-cracked gas reservoirs, and (3) a “reservoir densification precedes hydrocarbon accumulation” type of late-stage kerogen cracking gas reservoirs
Summary
The efficient exploration and development of tight oil and gas resources in the 21st century has transformed the structure of the global energy (Zou et al, 2012; Wang et al, 2016). Multiple “marinecontinental transitional to continental facies” tight sandstone reservoirs have been discovered in Yanchang Formation of Ordos Basin, Xujiahe Formation of Sichuan Basin, and Ahe Formation of Tarim Basin and so on (Dai et al, 2012; Sun et al, 2019; Wang et al, 2016; Zou et al, 2012). The Dongpu Depression covers about 5300 km, which has attracted extensive attention from academia for being a typical representative of lowpermeability sandstone oil and gas enrichment of rift basins in East China (Figure 1(a) and (b)). Its internal structure is controlled by basement rifts, forming an east-west zoning structure, including the western slope zone, the western sag zone, the central uplift zone, the eastern sag zone and the Lanliao fault terrace zone (Figure 1(c))
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