Petroleum geology and exploration direction of gas province in deepwater area of North Carnarvon Basin, Australia

North Carnarvon Basin is a gas province with minor oily sweet spots in deepwater area with water depth more than 500 m,which is one of the hot spots of global petroleum exploration for its series of giant hydrocarbon discoveries in recent years.However,the degree of oil and gas exploration in deepwater area is still low,and the conditions for oil and gas accumulation are not clear.Based on the current exploration situation and latest database of fields,applying multidisciplinary analysis of hydrocarbon geology,hydrocarbon accumulation elements and its exploration direction of North Carnarvon Basin in deepwater area are analyzed.The results show that there are three sets of main source rocks in deepwater area of North Carnarvon Basin,which are Triassic marine shale in Locker Formation and delta coal-bearing mudstone with thin carbonaceous mudstone in Mungaroo Formation,Lower–Middle Jurassic paralic carbargilite and coal measure strata in Athol Formation and Murat Formation,Cretaceous delta mudstone in Barrow Group and marine shale in Muderong Formation.Most source rock samples show gas-prone capability.The coarse sandstone of delta facies in Middle–Upper Triassic Mungaroo Formation is the most important reservoir in deepwater area,Lower Cretaceous Barrow Group deep-water gravity flow or underwater fan turbidite sandstone is the secondly main reservoir.Lower Cretaceous marine shale in Muderong Formation is most important regional caprock.Triassic mudstone in Mungaroo Formation is an important interlayer caprock in deepwater area.There are two main reservoir accumulation assemblages in deepwater area,one is Triassic structural-unconformity plane reservoir accumulation assemblage of Locker Formation to Mungaroo Formation,and the other is Lower–Middle Jurassic Athol Formation and Murat Formation–Lower Cretaceous stratigraphic lithology-structural reservoir accumulation assemblage of Barrow Group to Muderong Formation.There are three main control factors of hydrocarbon Accumulation:One is coupling of source and seal control hydrocarbon distribution area,the second is multi-stage large wave dominated deltas dominate accumulation zone,the third is direction of hydrocarbon migration and accumulation in hydrocarbon-rich generation depression was controlled by overpressure.The south of Exmouth platform in deepwater area is adjacent to hydrocarbon rich depression zone,reservoir assemblage is characterized by“near source rocks,excellent reservoir facies,high position and excellent caprocks”,which is the main battlefield of deepwater oil and gas exploration in North Carnarvon Basin at present.There are a lot of fault block traps in the northern structural belt of Exmouth platform,and the favorable sedimentary facies belt at the far end of delta plain in Mungaroo Formation is widely distributed,which is the next favorable exploration zone.The Lower Cretaceous,which is located at the concave edge uplift adjacent to the investigator depression and the Exmouth platform,also has a certain exploration prospect in northwest of deepwater area.

Seismic interpretation and hydrocarbon accumulations implication of the Miocene Meishan Formation reefs in southern Qiongdongnan Basin, northern South China Sea

Numerous moundy reflections were found at the Upper Meishan Formation in southern Qiongdongnan Basin, South China Sea which are suspected reefs. However, no common understanding on the seismic and geologic interpretation was acquired. Based on seismic reflection identification, geometrical configuration description, wave impedance inversion, forward simulation, analysis of reef-building sedimentary environments and pseudomorph identification, these suspected-reef moundy reflections were studied in details.Three types of reefs were recognized, i.e., patch reef, platform-edge reef, and pinnacle reef. They have some typical characteristics of reefs, including moundy or lenticular shape, clear outline with crest and canal, internal foreset bedding, bidirectional uplap between limbs, and strong reflection at top and bottom. The patch reefs are of moundy seismic facies, with moderateestrong amplitude and moderate frequency, and distribute at the higher part of southern slope belt. Fore-reef and back-reef of patch reefs are slightly asymmetrical being perpendicular to palaeo-bathymetric orientation; while symmetrical to slightly asymmetrical being parallel to the palaeo-bathymetric orientation, and the wave impedance value is about 7 kg/m^3×m/s.Platform-edge reefs are of flat-moundy seismic facies with strong amplitude and moderate frequency, which develop at the carbonate platform edge of higher part of southern slope belt, and the wave impedance value is about 7.5 kg/m^3×m/s. Pinnacle reefs grow up on volcanic cones, forming the complex of pinnacle reef and volcanic cone. The Miocene Meishan Formation reefs in southern Qiongdongnan Basin, northern South China Sea, with high quality sourceereservoireseal assemblages, have good petroleum exploration prospects.