Research On Sedimentary Facies And Geologic Interpretation Of Moundy Reflectors Of Meishan Formation In Deepwater Area Of Qiongdongnan Basin

Nowadays petroleum in deepwater area is one of great fields of global petroleum exploration, the framework of global discovered oil and gas is "one vertical and one horizontal". Oil is mainly distributed in submeridional Atlantic offshore deepwater basins, gas is mainly located at sublatitudinal Neothetys structural domain offshore deepwater basins, and they are individually affected by "South-lacus North-marine North-South strike fault depressions" and "transitional facies East-West strike fault depressions". Reef reservoir has the superiority of good qualification of storage and high output, to witch a lot of attention was highly paid. Around South China Sea reef reservoirs are rich and play an important part, Reef is an important exploration realm in the deep-water areas of the northern South China Sea. In deepwater area of Qiongdongnan Basin, located in northern margin of South China Sea, the degree of reef reservoir exploration of Meishan formation is low, but its potential is great, we can certain if there is reef or not and confirm reef distribution characteristic by researching on sedimentary faces and geologic interpretation of moundy reflectors, and which are important references for the future study of reef reservoir forming conditions.In view of the actual situation of absence of drill holes in deepwater area of Qiongdongnan Basin, based on present situation and development of research at home and abroad, regional geology, data of drill hole,2D and3D seismic data, predecessors’outcomes, and so on, equitime sequence stratigraphic framework of Qiongdongnan Basin is set up by identifying the sequence interface of logging and seismic, making seismic synthetic records, and demarcation of horizon. By means of sedimentary facies analysis of single well and multiwell section, associated with formation thickness, seismic attribute and seismic facies analysis, sedimentary facies of upper and lower segment of Meishan formation is individually studied. With fine interpretation of seismic cross-section, particular description of geometric configuration, analog anlysis and forward modeling of geologic model, reasonable geologic interpretation of moundy is made, and its distribution characteristic and development pattern are also studied. Depend on the analysis of reef reservoir forming conditions, hydrocarbon accumulation pattern is summed up, and exploration potential of reef reservoir is Discussed.During the depositional stage of upper segment of Meishan formation in deepwater area of Qiongdongnan Basin, there were two big Northern provenances at Northern uplift zone, developing fan delta, delta, neritic facies and carbonatite platform. At central sag zone it existed three large depocenters, bathyal facies was the leading face with some slope fans at its north slope. There was a large unicline at Southern ramp region and Southern uplift zone, which declined towards North central sag zone, striking latitudinal direction, much more flatten than nowadays and lying in clear water environment. It mainly developed distributing Northeast to Southwest large patch reef group, wide area of carbonatite gentle slope and contourite mound, there was mainly large area carbonatite platform in Southern uplift zone, at platform edge there was a belt of platform edge reef, striking approach latitudinal direction and interrupt arrange. During the depositional stage of lower segment of Meishan formation the deposition feature was similar with the upper segment of Meishan formation at Northern uplift zone and central sag zone. Southern uplift zone, Beijiao bulge and part of Yongle sag were a unified flat carbonatite platform, Beijiao sag and Songnan low uplift were platform edge slope, developing carbonatite gentle slope deposition.The geologic interpretation of moundy reflectors of Meishan formation in deepwater area of Qiongdongnan Basin are reef, contourite mounds, slop fans and diapir structure. Platform edge reef, developing at carbonatite platform edge of South uplift zone, is strong amplitude medium frequency platy moundy seismic facies. Patch reef groups, mainly distributing at high placement of fault-dominated benchs, is middle to strong amplitude medium frequency moundy seismic facies. Patch reef groups and platform edge reef are distributing approach latitudinal direction at five fault-dominated benchs, five stage of sea level rising up sub-cycle controlled the development of five stage reef groups on five fault-dominated benchs. Contourite mounds, distributing approach Northeast to Southwest, mainly exist at low placement closing to fault which dominated the next higher bench of every fault-dominated benchs, which Surround the break of platform slope of fault-dominated benchs. Slop fans are middle amplitude medium frequency wedge or imbricated seismic facies, which mainly distribute at continental slope that between central sag zone with Northern uplift zone. Diapir structure, divided into volcanic diaper and muddy diaper, distribute scramble and illregular.In deepwater area of Qiongdongnan Basin the exploration foreground is broad, there is a lower source to upper reservoir-seal assemble with Eocene lacustrine facies mudstone and Oligocene Yacheng formation transitional facies coal measures with carbargilite are source rocks, upper segment of Meishan formation reef is reservoir and upper Miocene Huangliu formation to Quaternary Ledong formation thick bedded shale are seal. It exists two big hydrocarbon enrichment models:one is Southern uplift zone far source with unconformity long distant migration carbonatite platform edge reef hydrocarbon enrichment model, the other is Southern ramp region approach central sag zone with two source and fault short distant migration Patch reef groups hydrocarbon enrichment model.