Reservoir Characteristics and Genetic Mechanisms of the Mesozoic Granite Buried Hills in the Deep-water of the Qiongdongnan Basin,Northern South China Sea

Due to its structure,rock and mineral composition,fluid and other factors,the granite Buried Hill Reservoir is highly heterogeneous with a complex longitudinal structure and a reservoir space made up of a combination of dissolution pores and fractures.This paper is based on current understanding of tectonic evolution in the northern part of the South China Sea,in conjunction with the seismic phase characteristics.It is determined that the meshed fault system was formed by three stages of movement-tectonic compression orogeny during the Indochinese epoch,strike-slip compression-tension during the Yanshanian Period,early fracture extension activation during the Himalayan-which controlled the distribution of the Buried Hill Reservoir.Drilling revealed two types of buried hills,faulted anticline and fault horst,their longitudinal structure and the reservoir space type being significantly different.The mineral composition,reservoir space and diagenetic characteristics of the reservoir rocks and minerals were analyzed by lithogeochemistry,micro section and logging etc.,it thus being determined that the Mesozoic rocks of the Songnan Low Uplift in the Qiongdongnan Basin are mainly composed of syenogranite,granodiorite,monzogranite,which is the material basis for the development of the Buried Hill Reservoir.The content of felsic and other brittle minerals is more than 70%,making it easy for it to be transformed into fractures.At the same time,the weathering resistance of granodiorite and monzogranite is weaker than that of syenogranite,which is easily weathered and destroyed,forming a thick sand gravel weathering zone.With increasing depth of burial,weathering and dissolution gradually weaken,the deep acidic fluid improving the reservoir property of internal fractures and expanding the vertical distribution range of the reservoirs.The research results lay a foundation for the exploration of Buried Hill in the deep-water area of the Qiongdongnan Basin.

Genetic mechanism of the granite buried-hill reservoir of the Penglai 9-1 oilfield in Bohai Sea

The Penglai 9-1 oilfield is the largest granite buried-hill oilfield in China presently,genesis and evolution of the granite buried-hill reservoir is complex.Based on geochemical,geophysical,experimental simulation and other methods,and combined with field geological observation,genetic mechanism of the granite buried-hill reservoir of the Penglai 9-1 oilfield and its hydrocarbon accumulation mode were well investigated.Results showed that the granite was formed by magmatic intrusion along deep faults under intraplate breakup of North China Plate,it was the product of magmatic activities of Yanshan tectonic episode of Circum-Pacific Tectonic Region,and the intrusion time was 160e170 Ma of the Jurassic.Formation of the granite buried-hill reservoir was mainly controlled by the epigenic karstification and tectonic reconstruction,the Yanshanian weathering and denudation controlled macroscopic development characteristics of the granite buried-hill reservoir,and faults and joints formed by Cenozoic tectonic movement promoted modification of the granite buried-hill reservoir.Laterally,thickness of the granite buried-hill reservoir had a positive correlation with fracture density.Vertically,the granite rocks could be divided into five zones:soil zone,sandy zone,broken zone,fracture zone and base rock zone.The upper-middle part(sandy zone,broken zone,fracture zone)of the granite buried hill was the highquality favorable reservoir zone and the main oil-bearing interval.

Thermal and exhumation history of the Songnan Low Uplift,Qiongdongnan Basin:constraints from the apatite fission-track and zircon(U-Th)/He thermochronology

Significant advancements have been made in the study of Mesozoic granite buried hills in the Songnan Low Uplift(SNLU)of the Qiongdongnan Basin.These findings indicate that the bedrock buried hills in this basin hold great potential for exploration.Borehole samples taken from the granite buried hills in the SNLU were analyzed using apatite fission track(AFT)and zircon(U-Th)/He data to unravel the thermal history of the basement rock.This information is crucial for understanding the processes of exhumation and alteration that occurred after its formation.Thermal modeling of a sample from the western bulge of the SNLU revealed a prolonged cooling event from the late Mesozoic to the Oligocene period(~80-23.8 Ma),followed by a heating stage from the Miocene epoch until the present(~23.8 Ma to present).In contrast,the sample from the eastern bulge experienced a more complex thermal history.It underwent two cooling stages during the late Mesozoic to late Eocene period(~80-36.4 Ma)and the late Oligocene period(~30-23.8 Ma),interspersed with two heating phases during the late Eocene to early Oligocene period(~36.4-30 Ma)and the Miocene epoch to recent times(~23.8-0 Ma),respectively.The differences in exhumation histories between the western and eastern bulges during the late Eocene to Oligocene period in the SNLU can likely be attributed to differences in fault activity.Unlike typical passive continental margin basins,the SNLU has experienced accelerated subsidence after the rifting phase,which began around 5.2 Ma ago.The possible mechanism for this abnormal post-rifting subsidence may be the decay or movement of the deep thermal source and the rapid cooling of the asthenosphere.Long-term and multi-episodic cooling and exhumation processes play a key role in the alteration of bedrock and contribute to the formation of reservoirs.On the other hand,rapid post-rifting subsidence(sedimentation)promotes the formation of cap rocks.