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

Abstract

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.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019