中国南方志留系油气地质特征与勘探方向

Geological features and exploration direction of the Silurian hydrocarbon reservoirs in Southern China

中国南方志留系勘探程度较低,制约了对志留系自身油气成藏特点的研究和认识。从烃源岩的时空分布、优质烃源岩的发育环境、生烃史3个方面分析了中国南方上奥陶统—志留系的烃源条件;分析了志留系发育碎屑岩、碳酸盐岩两类储层的条件,对中国南方志留系油气成藏的基本特点进行了总结。该区上奥陶统—志留系优质烃源岩主要发育于上奥陶统五峰组和下志留统龙马溪组底部,以好—最好烃源岩为主,川东南—鄂西渝东区上奥陶统—下志留统烃源条件最好。上述两类储层均以低孔低渗—特低孔低渗为特征,其中,碎屑岩是主要的储集岩类。下志留统龙马溪组为优势盖层发育层位,是一套良好的区域性盖层。扬子区志留系泥岩具有较低孔隙度、较低渗透率、高突破压力、以超微孔为主的物性和孔隙结构特征,现今演化程度虽高,但仍具有良好的封盖能力。志留系天然气具油型气特征,并可能与储层中原油裂解成气有关,至少具有2期油气生成和运聚过程。结论认为:川东南—鄂西渝东区为志留系天然气勘探最有利区,江汉平原南部冲断褶皱区和下扬子海安地区则为勘探有利区。

The research and understanding of the Silurian hydrocarbon pooling patterns in Southern China are hampered by the low level of exploration there.This paper analyzes the conditions of the Upper Ordovician-Silurian source rocks in Southern China in respect of their temporal and spatial distribution,sedimentary environment of quality source rocks,and hydrocarbon generation history.It also studies the forming conditions and the basic patterns of the Silurian hydrocarbon reservoirs in Southern China.The Upper Ordovician-Silurian source rocks with good quality mainly occur in the Upper Ordovician Wufeng Formation and the bottom of the Lower Silurian Longmaxi Formation,dominated by good and most favorable source rocks.The source rocks in southeastern Sichuan Basin,western Hunan and eastern Chongqing are the best.There are two types of reservoirs developed in the Silurian period,namely,clastic reservoirs and carbonate reservoirs,of which the former is predominant.Both types of reservoirs are characterized by low to extra-low porosity and permeability.The cap rocks with high quality are developed in the Lower Silurian Longmaxi Formation,and act as a good regional seal.The Silurian mudstones in the Yangtze area are featured by relatively low porosity and permeability,high breakthrough pressure,as well as the dominance of extra-micro pores;although being at a high level of evolution,they still have relatively good sealing capacity.Natural gas from the Silurian-age reservoirs is of oil-type and may relate to the oil-cracking gas origins in the reservoirs.The hydrocarbons were generated,migrated and accumulated at least in two periods.It is suggested that the southeastern Sichuan Basin,western Hubei and eastern Chongqing are the most favorable areas for exploration of the Silurian gas reservoirs,while the thrust-fold area in the southern Jianghan plain and the Hai'an in the lower Yangtze area are also favorable.