Features Of The Discharging Zones Of Overpressure System And Its Coupling Relationship With Gas Migration And Accumulation In The Qiongdongnan Basin, South China Sea

Qiongdongnan basin is rich in petroleum resources which located in northern South China Sea and typically characterized by high temperature and high pressure. The Superposition of the multi-episodic rifting events occured in the early stage and the rapid deposition and structural subsidence took place at the later stage led to a unique environment of temperature and pressure, and so the burial history. The processes of petroleum migration and accumulation are quite complicated. There has been no breakthroughs for a long time in the exploration of Qiongdongnan basin since the findings of YC13-1gas feild, YC13-4and YC13-6gas reservoir around the Yanan depression, although some gas beddings and gas-bearing structures were found around the Yanan depression or in the eastern basin. The primary cause may be the less understanding of the law of petroleum accumulation under overpressureed environment, especially the relationship between the distribution of hydrocarbon and the overpressure system. The development of overpressure would not only cause hydraulic ruptures in the cap rocks which leading to the loss of oil and gas accumulated in the early stage, but also change the migration phases and dynamics of the natural gas. Under high pressure and high temperature conditions, the gas would migrate in water-soluble phase and come to the failure of hydraulic sealing in caprocks. Thus, only by degasification can the water-soluble gas accumulate.Thus it can be seen, the zones with rapid pressure decreasing which is also called pressure discharging zones, locating at the edge or inside of the overpressure system, were the favourable sites for hydrocarbon accumulation. This study began with the microcosmic and macroscopic geology characteristics of the pressure discharging zone, distinguished and divided the discharging zone based on the diagenetic features and logging response, studied the macroscopic distribution and evolution by integrated application of overpressure prediction and basin modeling technology, furtherly discussed the coupling relationship between pressure discharging zone and hydrocarbon accumulation, then established a petroleum accumulation model which considered the degassing and accumulation of water-soluble gas and with a certain predictive feature. The main research contents and realizations achieved in this article are as follows:(1) Identify the pressure discharging zone according to the microscope information of fluid activities revealed by the diagenesis occured in the sandstone reservoir. By observation of thin sections and cathdoluminescence, it was found that the siliceous cementation phenomenon were very common in the Yacheng area and there may exist two stage of cementation, the first stage was mainly the cementation fillings in cracks, the second stage was mainly the quartz overgrowth which was more obvious than the first stage. The observed quartz overgrowth at the YC13-1structure was mainly in the Lingshui and Yacheng formations while at the YC35-1structure it mainly located at the bottom of the second member of Huangliu formation, the thickness of reservoir sections with quartz overgrowth were usually more than200m. Though the phenomenon of quartz overgrowth were also found in the wells distributed along the No.2fault, the setions were much thinner than that in west areas and mainly situated in the Lingshui and Sanya formation. The sections with quartz overgrowth phenomenon usually had good reservoir physical property (porosity and permeability) and were more common in western areas. By observation under microscope, we can see that the high porosity was mainly caused by the dissolution of detrital grains. Massive development of the quartz overgrowth and the dissolution pore usually means massive hot acid fluid flow and indicates the pressure discharging zone where the gas degassing occurred and large amount of water-soluble gas accumulated.In addition, the albitization of K-feldspar were also observed to frequently happen in the quartz overgrowth sections in the deeper layers, and the albite formed by metasomatism usually take a shape of thick overgrowth around the K-feldspar. Because this phenomenon was suggested only appear in the late stage of diagenesis, it may indicate the migration of high temperature fluid. The abnormal changes of clay minerals can also indicate the pressure discharging zone, it was usually caused by fluid flow near the pressure discharging zone. The maturity indictors of organic mater can also be used to identify the short-term thermal anomaly near the pressure discharging zone.(2) The structure of pressure system in Qiongdongnan basin can be divided into three types: normal pressure, transmission overpressure and compound overpressure, accordingly, four types of pressure discharging zones were indentified. The normal pressure system can be subdivided into a normal compaction section and a stable velocity section, the latter one mainly distributed from the bottom of Yinggehai-huangliu formation to the Meishan formation in the shelf area from the northen of No.3fault, thinning in the direction of continental slope. The transmission overpressure system distributed along the No.1fault zone in the western area and around the overpressure system in the central depression zone. The compound overpressure system exists widely in the central depression. The type I pressure discharging zone distributed only along the No.1fault zone and generally located in the bottom of the Ying-huang formaion, this type was not suitable for hydrocarbon accumulation for its bad conditions. The type Ⅱ pressure discharging zone also distributed along the No.1fault zone, and acted as the most important one among the four types around the Yanan area. Large amount of siliceous cementation were found in the reservoir sandstone, indicating that type Ⅱ was more favorable for petroleum accumulation. The type Ⅲ pressure discharging zone which located on top of the deep compound overpressure system was very common in the central depression. The type Ⅳ pressure discharging zone was located inside the deep overpressure system, great amount of quartz overgrowth developed locally in some sections inside the overpressure system, indicating that this type was also favorable for petroleum accumulation but not a primary type.(3) The distribution of the overpressure system in Qiongdongnan basin was predicted by integrated application of the seismic velocity data and pressure measurements, on the basis of logging analysis. The influence of the stable velocity section which widely developed in the Yanan area should first be calibrated. According to the calculation, many beaded overpressure centers along the continental slope had developed in the deep Central depression, they were not isolated but gradually transited into a huge overpressure system. The overpressure system were stronger along the continental slope and weaker on the continental shelf, and were stronger in the west than in the east, this may suggest that the quickly southward movement of the continental slope during the late accelerated deposition stage and the overpressure transmitting from the Yinggehai basin had significant influence on the present overpressure pattern and its evolution. The top of overpressure system in the Ledong-Lingshui sags were mainly below the depth of2250～2500m (within Yinggehai formation), while in the Songnan sag it was mainly below2500m (within Huangliu formation), in the Baodao sag it was mainly below2500m (within Meishan and Sanya formation), and in the Changchang sag it was mainly below4500m (within Sanya formation).(4) The evolution characteristics of the paleo-pressure field were studied by the application of numerical simulation methods. A lot of work referring to the model research and parametric statistics should be taken first. According to modeled results, the overpressre system in the central depression zone had already formed at the late stage of Yacheng formation, there were four overpressure centers located in the Ledong, Lingshui, Baodao and Changchang sags, with a certain isolation from each other. The overpressure systems got much strong during the stage of the second member and the first member of Lingshui Formation, then after several cycles of pressure evolution, a huge overpressure system much stronger than ever gradually formed in the west basin, with a strongest overpressure center in the Ledong sag and a trend transmitting from the west to the east. The earliest pressure discharging event occurred during the late stage of Yacheng formation and the late stage of the third member of Lingshui formation, and it’s more obvious in west. The second pressure discharging event, which was much stronger, happened during the late stage of the first member of Lingshui formation and the late stage of the second member of Sanya formation. The third pressure discharging event, much stronger than ever, took place in late stage of Meishan formation. The fourth pressure discharging event, which was much weaker, happened during the stage of Huangliu formation, and continued to the current time in the eastern. (5) The spatial and temporal distribution of the pressure discharging zones were analyzed based on the prediction of current overpressure system and the modeled evolution of paleao geopressure field. According to the prediction results, the top surface of current overpressure system in the central depression just deepens toward both the northern and southern slopes, and the overpressure changes much faster in northern slope, indicating that pressure discharging condition in the north slope was much better, as well as the petroleum accumulation condition. This should be related to the contribution of fault-sand transmitting systems developed on the northern slop. According to the modeling results, several sites remained at the edge of the overpressure system during the evolution of overpressure system, they were Songnan low uplift, Beidiao uplift, the easterm of Lingnan low uplift, the southern of Yabei area and the southern slope of Songtao uplift, these sites should be the potential pressure discharging zones.(6) The primary stage of natural gas accumulation could be synthetically determined by modeling of the maturation history of organic matter, the hydrocarbon generation and expulsion histories of main source rocks, combined with the charging events revealed by fluid inclusion, it’s concluded that the primary gas accumulation stage should since from10.5Ma. On the one hand,10.5Ma was during the peak gas generation stage of the source rocks in Lingshui formation on the slops of the Central Depression Zone, while the source rocks in Yacheng formation still generated massive hydrocarbon, thus massive gas could dissolved in the deep formation water in the high temperatures and high pressure environment. On the other hand,10.5Ma was at the beginning of the forth pressure discharging event, energy release of the overpressure system should cause a large number of mass transfer and temperature decreases, this was propitious to the segregations of deep water soluble gas migrating along the pressure discharging zone. In addition,10.5Ma was a little earlier than the hydrocarbon charging period determined by the fluid inclusion, which seems reasonable.(7) Analyze the coupling realationship between the evolution of pressure discharging zones and the gas accumulation. Accumulation of water soluble gas in the pressure discharging zones was totally determined by pressure, temperature and the amount of dissolved gas, whether enough amount of dissolved gas and enough changes in solubility were very important. The favorable discharging zones should be those not only link the deep water soluble gas and the shallow reservoir, but also cause significant changes in the temperature and pressure conditions. Relatively, the accumulation conditions of type II discharging zone was optimal, thus had more active fluid activity. The favorable sites of type III discharging zone were those near the Lingshui low uplift and the northern slop of Baodao sag, along the boundary faults and close to the normal pressure zone.