The Characteristics Of The Upper Sinian Dengying Formation Reservoir Rocks In The Southwestern To Central Sichuan Basin

As far as the Sinian Dengying Foramation is concerned，the southwestern tocentral Sichuan is the most important gas province in Sichuan Basin. However,previous research paid few attention to the diversity and comparative study of reserviorcharacteristics among different regines, not revealing fully the main controlling factors.Based on the depiction and comparative studies of the sedimentary, diagenetic, andreservior characteristics among Jinshi, Weiyuan-Ziyang, and Chuanzhong regine, thisstudy tried to promulgate the essence that lead to the Deying Formation reserviorcharacteristics’ difference, and the main controlling facrors of the high-qualigyreservior in the study area.The Sinian Dengying Formation have stable tidal flat sedimentary environment,which was benifacial to the cyanobacteria’s growth.The Second and the Forth Sectionof Dengying Formation developed kinds of visibale and invisible cyanobacteria whichtook part in the sedimentary and diagenetic process, accumulating microbialitedolostone that is the vechcle of reservior in this area.There were18kinds of microfacies in the Dengying Formation. And thesecomposed4types of meter-scale sedimentary circle shallowing upward in the Secondand the Forth Section of Dengying Formation: Circle â… was the subtidal zone-intertidalzone-supratidal zone; Circle â…¡ was the subtidal zone-intertidal zone; Circle â…¢ was theintertidal zone-supratidal zone; Circle â…£ was subtidal mound-bank compelx or grainbank coarsening-upwards. The Third Secion developed mixed rock, because when itdeposited much terrigenous detrital sediment input that went against thecyanobacteria’s growth.The Dengying Formation underwent2times of transgressive-regressive cycle.Denger Section deposited in the first regressive cycle, developing mutiple Circle â… andCircle â…¡. Each circle had thick subtidal deposition and thin surpratidal deposition, orlack surpratidal deposition. The Dengsan Section to the middle of Dengsi Sectiondeposited in the second transgressive cycle, becoming clear upwarding and developingCircle â… , Circle â…¡, and Circle â…¢. The upper part of Dengsi Formation deposited in the second regressive cycle, developing mainly Circle â…¢. And every cirlce had thickintertidal and supratidal depositon, and thin subtidal tital or lack it. According tomicrofacies and the evolution of sedimentary facies, the microbial carbonate tidalsedimentary model and mixed tidal sedimentary models were set up.The diagenesis of the Sinian Dengying Formation included microbialdolomitization, cementation, compaction, silication, recrystallization, dissolution,fulling, and fracturing. Its diagenetic sequence was followed as below:dolomitizationâ†'cementationâ†'contemporaneous karstification/silicationâ†'compaction/recrystallization/fillingâ†'karstification/fracturingâ†'fillingâ†'pressuresolution/recrystallizationâ†'organic-acid dissolution/hydrothermal actionâ†'fillingâ†'fracturingâ†'filling. But there were some differences in these3regions because of theirdifferent tectonic position.There were7types of reservior space in Dengying Formation. And these were:(1)corrosive vugs and caves;(2) grape-lacelike holes and caves;(3) inter microbialframework pores;(4) intercrystal pores;(5) intergrain pores;(6) intragrain pores andmodic pores;(7) fractures. Most of them were secondary origin, which suggested thatDengying Formation was mainly consisted of secondary reservior space when it wentthrough long time diagenesis.The bitumen content in the Dengying Formation’s pores, caves, and fracturesranged from1%to4%, except for that of Jinshi region. In general, the bitumen contentof Dengying in Jinshi and Weiyuan-Ziyang regions become higher with the distance toits top being closer. The bitumen content in Chuanzhong region from the DengerSection to the Dengsi Section also showed increasing trend, and that of Dengsi Sectionstill increased. However, the change rule of that in Denger Section was not obvious. Allof these indicated that the Dengying Formation reserviors in Chuanzhong region wasunder control by karstification just being as same as those in Jinshi andWeiyuan-Ziyang regions. But the reserviors controlling factors of Denger Section weremuch complecated than those of Dengsi Section.The Dengying Formation reserviors physical properties in Chuanzhong regionwas superior to those in Weiyuan-Ziyang region which had an advantage over those inJinshi region. That was because that the early CambrianMianyang-Lezhi-Longchang-Changning intracratonic sag controlled the intensity ofkarstification, hydrothermal action, liquid hydrocarbon filling. Being adjacent to theintracratonic sag, these3types of diagenesis were stronger. So the distance to theintracratonic sag were cloer, the Dengying Formation reserviors physical properties were better.The high-quality reserviors main controlling factors of Dengying Formation in thesouthwestern to central Sichuan Basin included factors such as:(a) favorablemicrofacies controlled the potential of turning into high-quality reserviors;(b)karstification was the key to high-quality reserviors formation;(c) strong hydrothermalaction improved effectively reserviors;(d) fracturing could increase the permeability;(f)the efficient filling of liquid hydrocarbon was the key to the reservoirs preservation.