Controlling Factors Of Source Rock Development And Distribution In The Middle-upper Permian,Northeastern Sichuan Basin

Several large-medium sized gas fields,such as Puguang and Yuanba,have been found in Changxing Formation and Feixianguan Formation in the northeastern Sichuan Basin.Previous studies suggested the gas was originated from source rocks of the Upper Permian Wujiaping Formation developed in“Kaijing-Liangping”shelf.With the advancement of exploration,organic-rich shales of the Middle Permian Gufeng Member and the Upper Permian Dalong Formation have been revealed by some new wells,indicating possible significant contributions to hydrocarbon accumulation.However,there is still no systematic study for these source rocks on comparing their geochemical characteristics,distribution rules,organic matter enrichment and preservation conditions.Based on core and field outcrop observations,combined with a large amount of logging data,and guided by principles of sedimentary petrology and geochemistry,this paper clarified main controlling factors of source rocks development and distribution,analyzed the enrichment mechanism of organic matter.Four conclusions have been obtained.（1）Three stages of intra-platform basin facies deposits developed in northeastern Sichuan Basin during Middle-Upper Permian.The first stage was formed when the global sea level fell to the lowest point,accompanied by basement faults activation caused by the Emei tafrogeny,namely the Kaijiang Liang intra-platform basin,where the Gufeng Member deposited,mainly consisted of siliceous rocks,siliceous mudstones,and limestone.The second stage was formed when the sea level began to rise after the uplift and denudation at the end of Gufeng Member.The intra-platform basin deposited marine-continental transitional coal strata in the early-middle Wujiapingian and marine lime mudstone with limestone layer.The third phase was formed during the sea level continued to rise with basement faults activation caused by the South Qinling Ocean expansion.The intra-platform basin subsided and became to be the deep-water shelf,where deposited the biological siliceous rock,shale and limestone of the Dalong Formation.（2）The intra-platform basin is most favorable for the development of high-quality source rocks of the Gufeng Member,the Wujiaping Formation and the Dalong Formation in northeastern Sichuan,followed by the slope facies.The sedimentary center of three sets of high-quality source rocks in the intra-platform basin was in the Puguang area,with an average cumulative thickness about 50～60m.The high-quality source rock of slope facies were mainly developed in the Yuanba area,with a thickness about 20～40m.（3）The development of Gufeng Member was controlled by the short-term rapid subsidence of the intra-platform basin caused by the activation of basement faults,forming a set of very thin source rock.The Wujiaping Formation and the Dalong Formation were mainly controlled by the influence of relative sea level rise.In the early-middle Wujiapingian,coal-bearing source rock with type III organic matter was developed in the inra-platform basin.In the late Wujiapingian,the water deepened with sea level rising,and source rocks with type II₁ organic matter were developed,with an average TOC of 2.82%.The water depth in the intra-platform basin reached maximum during the deposition of Dalong Formation,and source rocks containing type II₁ organic matter were developed,with an average TOC of 5.27%.（4）The mechanism of organic matter enrichment and preservation has been revealed.The Gufeng Member was deposited in a relatively deep basin with limited circulation,and due to the hydrothermal fluid activity,the nutrition and paleoproductivity of the water body were improved.Since the limitation of the water and the consumption of oxygen during a large amount of organic matter degradation,euxinic strong reduction environment was formed.In the late Wujiapingian,as sea level continued to rise,medium-high paleoproductivity and strong reduction environment were formed in the intra-platform basin.The Dalong Formation was deposited in deepwater shelf.Upwelling carried nutrients and led to high paleoproductivity.The low-energy environment and consumption of oxygen during a large amount of organic matter degradation caused euxinic strong reduction environment.In conclusion,there are three sets of high-quality source rock in northeastern Sichuan Basin.The Dalong Formation has the largest thickness and the highest hydrocarbon generation potential,which should be the main hydrocarbon source of Puguang and Yuanba large gas fields,as well as the most potential formation for shale gas exploration in northeastern Sichuan Basin.