--> Geochemical Characteristics and Application of Sterane and Terpane Biomarkers in Heavy Oils From Lacustrine Source Rocks of Liaodong Bay, Bohai Bay Basin, China

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Geochemical Characteristics and Application of Sterane and Terpane Biomarkers in Heavy Oils From Lacustrine Source Rocks of Liaodong Bay, Bohai Bay Basin, China

Abstract

Liaodong Bay is located in the northeast of Bohai Bay Basin of China and covers an area of 12500 km2. Over 60% of the oils discovered are heavy oil (ρ≥0.934g/cm3, 20°C). LD21, LD5 and JZ23 heavy oil fields were discovered in last three years. The heavy oils have shallow burial depth ranging from 800m (2625ft) to 1800m (5905ft) with high density ranging from 0.935 to 1.011 g/cm3 (20°C) and high viscosity ranging from 238.2 to 74462 mPa s (50°C). The heavy oil production of LD21-1A well can reach up to 87.28 m3/day demonstrated by drill stem test. There are three organic-rich source rock intervals: the third and the first member of Shahejie Formation (Es3, Es1) and the third member of Dongying Formation (Ed3). The TOC of Es3 source rock can reach up to 8.88%, which is type II kerogen dominated. The biomarkers indicate that Es3 source rock is deposited in freshwater, anoxic environments with minor terrigenous organic matter input. The TOC of Es1 source rock can reach up to 5.62%, which is type I and II kerogen dominated, and it is deposited in saline, highly reducing conditions with minor or no terrigenous organic matter input. Oils generated from Es3 and Es1 source rock have relatively low C19/C23 tricyclic terpane (C19/C23 TT), C24 tetracyclic terpane/C26 tricyclic terpane (C24 TeT/C26 TT). However, Oils generated from Es3 source rock have high Ts/Tm but low ETR [ETR=(C28+ C29)/(C28+ C29+Ts)]. Oils from Es1 source rock display low Ts/Tm but high ETR. The TOC of Ed3 source rock can reach up to 4.74%, which is type II and III kerogen dominated, and it is deposited in freshwater, suboxic environments with substantial terrigenous organic matter input. Oils from Ed3 source rock have relatively high C19/C23 TT, C24 TeT/C26 TT and Ts/Tm but relatively low ETR. The tricyclic and tetracyclic terpanes, ETR and Ts/Tm can be used to distinguish oil source of the heavy oils (oil biodegradation level ranges from 6 to 9 according to criteria by Peters and Moldowan, 2005) and source rocks deposited in different sedimentary environments, as they are seldom influenced by biodegradation. These biomarkers are successfully applied in the LD21 oilfield with proven oil reserves over 100 million tons which located in the inversion zone of the south sag. Shallowly buried heavy oils with low C19/C23 TT, C24 TeT/C26 TT and ETR but high Ts/Tm are from Es3 source rock. Oils migrated along the adjusting faults of the strike-slip fault and accumulated in the Neogene during the last 12Ma.