The coupling relationships of the Oligocene fault fan system and their control over natural gas accumulation in the Qiongdongnan Basin

Abstract

Oligocene (fan) delta-associated coal measures and marine source rocks are the main gas-generating source rocks in the Qiongdongnan Basin, and (fan) delta sand bodies are the dominant types of reservoirs. By comprehensively utilizing a large amount of 2D and 3D seismic data and new drilling data, the distribution of (fan) deltas and three types of Oligocene fault fan systems were systematically identified; these are thought to have had joint control over the development of hydrocarbon source rocks and reservoirs. The three types of Oligocene fault fan coupling relationships in the Qiongdongnan Basin were found to be fault bend, fault gully, and fault tip. The fault bend type of (fan) delta developed at the bend of the fault and was distributed in the downthrown block of Fault Nos. 5 and 2. The fault tip type developed where the fault died out toward the source direction and was distributed near the east–west faults on the north and south sides of Baodao and Changchang Sags. The fault gully type developed along the fault trend in the downthrown block and was mainly distributed in areas including the northern part of Ya'nan Sag and the Fault No. 3 downthrown block. The fault bend (fan) delta is surrounded by a relatively closed bay environment, and the terrigenously dominated marine source rocks in the still water bay environment were developed with a high hydrocarbon generation index and high hydrocarbon generation potential. Fault tip and fault gully types of (fan) delta plain facies have a wide distribution of plain subfacies, which is conducive to the development of coal-measure source rocks that have excellent gas potential. During sedimentation of the fault gully and the fault tip delta, the terrain was relatively flat, with sand bodies strongly reworked by hydrodynamic force; the reservoir quality of the sand bodies in the underwater distributary channel and estuary dam are good, and the sand bodies have wide distribution. The sand bodies of the fault bend type of (fan) delta were deposited rapidly near the source kitchen, and the degree of sorting and rounding was poor, which resulted in the poor physical properties of the plain subfacies. In the southern and northern step-fault zones of Baodao Sag in the eastern part of this basin, the fault bend, fault tip, and fault trench (fan) deltas were inherited and further developed, and the source-reservoir configuration conditions were favorable. Exploration deployment was carried out in the Baodao 21 area in the northern step-fault zone and the Yongle 10 area in the southern step-fault zone, where new discoveries in natural gas exploration have been realized. In the eastern part of this basin, the fault bend, fault tip, and fault gully (fan) deltas in the southern and northern fault step zones of Baodao Sag were successively developed, which were beneficial for favorable source–reservoir configuration conditions. The natural gas exploration successes in the Baodao-21 area in the northern fault step zone and the Yongle-10 area in the southern fault step zone are beneficial for new discoveries in natural gas exploration in the Qiongdongnan Basin.