Study On Hydrocarbon Accumulation Mechanism In Huizhou Sag

Huizhou Sag is the best rich hydrocarbon generating sag in Zhu I Depression of Pearl River Mouth Basin, which have been proven by the exploration and following petroleum production. More than 10 oil and gas fields have been found within Huizhou Sag and make a great contribution to the 13 consecutive years of annual output of over 10 million square. However, because of the short history of oil and gas exploration and low study degree, the research on hydrocarbon generation,migration, accumulation and preservation process is still unclear today, which restricts the targets selection and future exploration, all these need an urgent deepen and more clear research for the future.In this paper, by the guidance of modern petroleum geology theory, we have studied the mechanism of hydrocarbon accumulation in Huizhou Sag from several aspects, such as the petroleum geological elements, dynamics of petroleum accumulation and rules of hydrocarbon accumulation. This study,to some extent,enriches and improves the content of hydrocarbon accumulation of Huizhou Sag. The main understanding of study were as follows:1. By the guidence of research thinking of the petroleum system and based on the collected geological information, the petroleum geological elements of Huizhou sag have been studied, including source rocks, reservoir, seal, source-reservoir-seal assemblage, trap and migration pathway system. The hydrocarbon in Huizhou Sag was generated from the two sets of source rocks, one group is the Wenchang good-very good source rocks, which is the main source rocks in the study area; the other group is Enping good-medium source rocks, which is the assistant set. The hydrocarbon generating threshold of these source rocks is about 2750m. Multiple reservoirs are developed in Huizhou sag, oil and gas are mainly distributed in reservoirs of Zhuhai-Zhujiang Formation,which is characterized by high-medium porosity and high-medium permeability, the Hanjiang, Enping and Wenchang Formations are the secondary or potential reservoir. The Hanjiang, Zhujiang and Wenchang Formations are the three regional seals in the study area, which are characterized by the wide distribution and thick cumulative thickness; the lower member of Zhujiang Formation together with Enping Formation are the regional seals and Zhuhai Formation actes as the local seal. There are three major regional source-reservoir-seal assemblages in the study area. The upper and middle member of Zhujiang Formation with lower member of Hanjiang Formation is upper assemblage, Zhuhai and Enping Formation is middle assemblage, Wenchang Formation and Pre-Tertiary is lower assemblage. Various types of traps are recognized in Huizhou sag,traditional anticline traps have a large proportion in all traps which have been drilled in, however, the non-anticlines traps have great petroleum potential in the future exploration. In the study area,faults and fractured type,transport layer type, unconformity type and complex migration system connected the generating hydrocarbon and appropriate potential oil and gas field, in which the vertical migration by fault is the most important migration system.2. Based on various kinds of measured data, guided by the theory of fluid dynamic field, with the application of basin modeling technology and reservoir geochemistry technology, the dynamics of petroleum accumulation conditions in Huizhou Sag are selectively analyzed and evaluated. The geothermal temperature field was conducted and the results indicated that the whole depression has the normal geothermal temperature field,the main temperature gradient distributes between 2.9-4.3℃/100m, the positive temperature gradient anomalies are founded in hydrocarbon generative subsags and high structural positions; the ancient geothermal gradient was higher than that of present, the temperature gradient evolution had high-low-high regularity, reflecting the evolution stage of Huizhou Sag. The high ancient geothermal gradient was favorable for hydrocarbon generation and migration, and the present normal geothermal gradient is helpful for hydrocarbon preservation. The pressure field was conducted and the results indicate that, the present pressure is characterized by normal pressure within most of study area, overpressure only developes in Enping and Wenchang Formation. In addition, the pressure evolution has close connection with the thermal evolution of organic matter. With the increase of thermal evolution of organic matter, pressure in source rocks increased step by step until overpass the threshold of critical pressure, then oil and gas expelled from source rock by episodic expulsion, finally, pressure decreased to normal level and then the next circle began. Therefore, Wenchang Formation source rocks in deep subsags generated significant overpressure as the organic matter reached high thermal evolution degree in the end of Hanjiang formation deposition, Enping formation only produced a low residual pressure in the end of Yuehai formation deposition. By contract, no overpressure was developed within the tectonic transition and the uplift zone. According to datas of fluid inclusion homogenization temperature, the oil and gas infill injection of Huizhou Sag could be divided into two phases, which happened during the deposition of the Miocene Hanjiang Formation and since Pliocene sedimentary period of Yuehai, respectively. The source rocks of south and west region in Huizhou Sag had higher maturity than the North region, therefore, infill injection of south and west region in Huizhou sag was earlier. The modeling of hydrocarbon migration history is finished and the results indicated that, the regional fluid potential has high value in deep sag and low value in uplift, this distribution has the trend:the regions near the Huizhou Sag and LF13 Subsag are main areas of flow supply and Huizhou-Lufeng Low Uplift, Central Huizhou Low Uplift, West Huizhou-Liuhua structure belt are fluid potential discharge areas, Thereinto, West Huizhou-Liuhua structure belt is the most favorable area for oil and gas accumulation.Based on reservoir geochemistry theory and research methods, the crude oil in research area can be classified into three groups according to their physical properties and geochemical characteristics. Oil of GroupⅠ, which is simply from Wenchang source rocks, mainly distributed in Dongsha Uplift, Huizhou-Lufeng Low Uplift, and XJ30-2 Oilfield; Moreover, Oil of GroupⅡ, mixed oil derived from multiple source rocks of Wenchang and Enping source rocks,is found in the uplifts within the Huizhou Sag; the last group only discovered in well of HZ9-2-1,which is simply from Enping source rocks.3. The geologic characteristics of reservoirs are depicted based on the petroleum geology and geochemical datas combined with basinmod methods, the hydrocarbon accumulation conditions and process are discussed by depicting HZ25-7 Oil-bearing structures, HZ21-1 buried hill structure, LH11-1 Oilfield, LF13-2 Oilfield, and then the corresponding hydrocarbon reservoir formation models are finally summarized. Analysis on hydrocarbon accumulation conditions and rules of Huizhou-Liuhua and Huizhou-Lufeng hydrocarbon accumulated zones are made on this basis,the hydrocarbon accumulated models could be divided into four types:close distance-single source-fault vertical migration-multi-stage accumulation model, close distance-mixed source-fault vertical migration-multi-stage accumulated model, close distance-mixed source-vertical and lateral migration-multi-stage accumulated model, far distance-mixed source-lateral and vertical migration-multi-stage accumulated model; In the contract, Huizhou-Lufeng hydrocarbon accumulated zone has a simple model of close distance-single source-vertical and lateral migration-episodic rapid accumulation model.