Characteristics And Main Controlling Factors Of Hydrocarbon Accumulation In The Eogene Of Huizhou Depression

Huizhou Depression is the best hydrocarbon-rich generating sag in Zhu I Depression of Pearl River Mouth Basin, which has been proven by the exploration and following petroleum production. More than 10 oil and gas fields have been found within Huizhou Depression and made a great contribution to the 13 consecutive years of annual output of over 10 million squares.However, the hydrocarbon is almost in the Neogene and Zhuhai Formation of Eogene, explorations and studies of deep part of the Eogene are less promising to recognize the characteristics and the main controlling factors of reservoirs. In this paper, by the guidance of modern petroleum geology theory, we summarize the characteristics and the main controlling factors of reservoirs in deep Eogene formation based on the studies of two aspects, one is geology factors and the dynamic process of accumulation, the other is the analysis of the reservoirs and comparison of hydrocarbon systems. Our data have shown some views which are beneficial for the research on hydrocarbon accumulation in deep Eogene formation.1. The petroleum geological elements of Huizhou Depression have been studied based on the collected geological information, including source rocks, reservoirs, seals, traps and migration pathway systems and according to the guidance of the petroleum system research. The hydrocarbon in Huizhou Depression was generated from two sets of source rock, the group one is the Wenchang good source rocks, which is the main source rock in the study area; the group two is Enping medium source rock, which is the secondary set. Both Wenchang and Enping reservoirs with low porosity and permeability, these deep reservoirs show poor in most areas, but reservoirs in Enping Formation are still good in the area with ancient uplift such as Lufeng and there is no regional seal except the local one. The good quality reservoir of Wenchang Formation is found as the result of sediment uncompaction and overpressure. Both regional and local seals exist in Wenchang Formation.2. Based on various kinds of measured data, guided by the theory of hydrocarbon dynamic, with the application of basin modeling technology and reservoir geochemistry technology, the hydrocarbon dynamic process in Huizhou Depression are selectively rebuilt. The results have shown:the geothermal gradient is between 2.9 and 4.3℃/100m. With down increasing deposition of layers, the depth increases, the heat flow and geothermal decrease. The initial geothermal gradient reached to 5.0℃/100m and decreased to 3.2℃/100m。Overpressure resulted from high speed sedimentation in early rift period, and it disappeared during the tectonic movement of Zhuqiong and Nanhai. Late high overpressure was caused by hydrocarbon generation.There are two hydrocarbon injection periods in Wenchang Formation, one is 15.8Ma-10.3 Ma, the other is from 7.5Ma to the present. Two periods exist in Lufeng area and western Huizhou Depression. They are 10.4 Ma and6.0 Ma in Lufeng,16.0-10.6 Ma and 8.8 -0 Ma in western Huizhou Depression. There is only one period for injection which is after 13.5Ma in Northern and Southern of Huizhou Depression.The whole depression was injected in 1-2 main periods, the episodic accumulation is beneficial for forming great reservoir. Although Wenchang and Enping Formations both are deep layers, their main control factors of reservoir are different.3. On the basis of analysis of reservoirs in Wenchang or Enping (HZ25-7,HZ21-1,LF7-1 and LF13-1) and comparison of three representative reservoirs (XJ24,HZ26 and LF13), we summarize the characteristics and the main control factors of reservoirs in deep Eogene formation. Source rock includes Wenchang and Enping Formations. Shale of deep lake facies in Wenchang Formation shows good quality, which is the main source rock. Shale of shore-shallow lake in Enping Formation shows poor, which is the secondary. The reservoir in Zhuhai Formation shows good quality, which is opposite to Enping and Wenchang Formations. In deep layer good reservoir formed in special area which is shallower and owns benefitable reservoir face. Local seal in Enping Formation and regional one in Wenchang Formation can serve the seal for hydrocarbon reservoirs. The types of the traps recognized in Enping and Wenchang are ancient buried hill, fault-anticline, anticline fault block etc. The main migarition pathways are faults and unconformities. In this paper the main controlling factors of deep layers we outlined are:source situation controls the accumulation of Enping source-Enping reservoir group; the fault situation is the main control of Wenchang source-Enping reservoir group; the quality of reservoir takes the most important role in accumulation of Wenchang Formation.