Reservoir forming conditions and key exploration technologies of Lingshui 17-2 giant gas field in deepwater area of Qiongdongnan Basin

Abstract

On September 15, 2014, China National Offshore Oil Co., Ltd announced that a high production of oil and gas flow of 1.6 × 106 m3/d was obtained in Well LS17-2-1 in deepwater area in northern South China Sea, which is the first great oil and gas discovery for self-run deepwater exploration in China sea areas, and a strategic breakthrough was made in natural gas exploration in deepwater area of Lingshui sag in Qiongdongnan Basin. Under the combined action of climax of international deepwater exploration, high oil prices, national demands of China, practical needs of exploration, breakthroughs in seismic exploration and testing technologies, innovations in geological cognition and breakthroughs in deepwater operation equipment, Lingshui 17-2 gas field is discovered. Among these factors, the innovation in reservoir forming geological cognition directly promotes the discovery. The quality of seismic data in the early time is poor, so key reservoir forming conditions such as effective source rocks, high quality reservoirs and oil-gas migration pathways are unable to be ascertained; with support of new seismic acquisition and processing technology, some researches show that Lingshui sag is a successive large and deep sag with an area of 5000 km2 and the maximum thickness of Cenozoic stratum of 13 km. In the Early Oligocene, the Lingshui sag was a semi-closed delta-estuarine environment, where the coal-measure and marine mudstones in Lower Oligocene Yacheng Formation were developed. The Lingshui sag is a sag with high temperature, and the bottom temperature of source rocks in Yacheng Formation can exceed 250 °C, but the simulation experiment of hydrocarbon generation at high temperature indicates that the main part of this set of source rock is still in the gas-generation window, with resources of nearly 1 trillion cubic meters, so the Lingshui sag is a hydrocarbon-rich generation sag. In the Neogene, the axial canyon channel from the Thu Bon River in Vietnam passed through the Lingshui sag, and five stages of secondary channels were developed in the axial canyon channel, where four types of reservoirs with excellent physical properties including the axial sand, lateral accretion sand, natural levee sand as well as erosion residual sand were developed, and lithologic traps or structural-lithologic traps were formed. The diapiric zone in the southern Lingshui sag connects deep source rocks in Yacheng Formation and shallow sandstones in the channels, and the migration pattern of natural gas is a T-type migration pattern, in other words, the natural gas generated from Yacheng Formation migrates vertically to the interior of the channel sandbody, and then migrates laterally in the channel reservoirs and forms the reservoirs. Innovations of geophysical exploration technologies for complicated geological conditions of deepwater areas are made, such as the detuning comprehensive quantitative amplitude hydrocarbon detection technology, which greatly improves the success rate of deepwater exploration; key technologies of deepwater safety exploratory well testing represented by the platform-dragged riser displacement technology are developed, which greatly reduces the drilling test cost. The above key exploration technologies provide a strong guarantee for the efficient exploration and development of Lingshui gas field. Keywords: Deepwater oil and gas, Source rocks, Lower limit of gas generation, The central canyon, Diapiric zone, Migration pathway, Lingshui sag, Lingshui 17-2 giant gas field, Qiongdongnan basin