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Charging of LD29 Oil Field on the Southern Slope Belt of Liaodong Depression from Multiple Lacustrine Source Rock Intervals and Generative Kitchens, Bohai Bay Basin, China


The southern slope belt of Liaodong depression has become a new key exploration area since the discovery of LD29 commercial oil field in 2016. The production of 1A well in LD29 oil field can reach up to 628.58 m3/day(oil) and 69484 m3/day(gas) demonstrated by drill stem test.

There are four organic-rich source rock intervals in the Liaodong Bay: the fourth, the third and the first member of the Eocene Shahejie Formation (Es4, Es3 and Es1) and the third member of the Oligocene Dongying Formation (Ed3). However, the Es4 source rocks are missing in Liaodong sag. Es4, Es3 and Es1 source rocks are characterized by low C19/C23 tricyclic terpane (C19/C23 TT) and low C24 tetracyclic terpane/C26 tricyclic terpane (C24TeT/C26TT). However, Es4 source rock displays low C27Dia/C27ST, medium 4-methyl steranes index(4-MSI) and gammacerane/C30-hopane(G/H). Es3 source rock displays medium C27Dia/C27ST, medium-high 4-MSI but low G/H. Es1 source rock displays relatively low-medium C27Dia/C27ST, low 4-MSI but high G/H. Ed3 source rock is characterized mainly by high C27Dia/C27ST, high C19/C23 TT, high C24TeT/C26TT, but low G/H and low 4-MSI.

The southern slope belt of Liaodong depression is adjacent to the Liaozhong sag and the Liaodong sag containing multiple mature source rock intervals. Biomarker associations of crude oil and source rock samples are analyzed to investigate the origin of oils on the southern slope belt. The crude oil samples of LD29 oil field display low C19/C23 TT (0.1-0.18) and low C24TeT/C26TT (0.49-0.53), indicating no or minor contributions from terrigenous organic matter. In addition, the oil samples have G/H ranging from 0.14-0.17, ETR[ETR=(C28+C29)/(C28+C29+Ts)] from 0.49-0.57, C27Dia/C27ST from 0.3-0.43 and 4-MSI from 0.3-0.36, displaying narrow variations. The biomarkers of the oil samples display relatively similar characteristics, therefore, oils from the southern slope belt of Liaodong depression should be divided into one class.

Based on the biomarkers and carbon isotopes analysis of the oil and source rock, it is concluded that the LD29 oils are mainly from the Es3 source rock of Liaozhong sag and Liaodong sag, with some contribution of the Es4 source rock of Liaozhong sag. The Eocene and buried hill area should be taken into seriously consideration in the future exploration on the southern slope belt, and the lithologic and stratigraphic trap in tectonic up-dip direction should be emphasized.