The Difference Of Hydrocarbon Accumulation Of Sinian And Lower Palaeozoic In The Middle And Southwest Of Sichuan Basin

The exploration in the Sinian-lower Paleozoic stratum have lasted for50years,and the Weiyuan gas field, Ziyang and longnvsi gas-bearing structures were discoveredduring this period. For a long period after the discovery of the Weiyuan Gas Field, fewprogress had been made on the exploration in the Sinian-lower Paleozoic stratum.Recently, two gas fields were found in Cambrian in Gaoshiti-Moxi structure, centralSichusan: the one was found in strata of Dengying Formation in the Sinian, the otherone was located in strata of Longwangmiao Formation. The current exploration anddevelopment shows that the gas accumulation in the Sinian-lower Paleozoic stratumdiffers fromstructureto structure. Onthe negative side, the degreeofgas charged ofthetrap in the Sinian stratain Weiyuan gas field was only25%, the present Ziyang structureis just agas-bearing structure,the SINOPEChad aunsuccessfulexploration in JS-1wellin Jinshi structure, which is close to the Weiyuan gas field. On the positive side, the gasfields found in Sinian and Cambrian in Gaoshiti-Moxi structure were found to be theextre-large gas fields. Based on the formation and evolution of theMianyang-Lezhi-Longchang-Changning intracratonic sag and the Leshan-LongnvsiPaleo-uplift, a lot of work about the correlation on the difference of the pool-formingin the Sinian-lower Paleozoic stratum were carried out, and a series of conclusionswere concluded.A horizontal comparison of the lithology of the Sinian reservoir, the type of theSinian reservoir space, the quality of the Sinian reservoir and content of bitumen of theSinian reservoir were applied among the three typical structures of Jinshi,Gaoshiti-Moxi and Weiyuan-Ziyang. The results of the horizontal comparison showedthat the reservoir quality in Dengying Formation in Gaoshiti-Moxiand Ziyang structureare better than that in Jinshi structure, because the reservoirs in Dengying Formation inGaoshiti-Moxi and Ziyang structure developed a large amount of denudation pores andcavities, and the the bitumen content ofthe pores and cavities were high. The amount ofdenudation pores and cavities developed in the reservoir in Dengying Formation inWeiyuan structure is relatively lower than that in Gaoshiti-Moxi and Ziyang structure,but the development degree of the cracks and permeability of the reservoir in Weiyuan structure are higher. The development degree of the cracks was low and the reservoirquality was poor in the reservoir in the Jinshi structure, and no bitumen was found inpores or cavities.The comparison and analysis on the diversity of the biomarker compounds of thebitumen filled in the Cambrian reservoirs among the different structures were carriedout. A oil-source correlation was applied between the biomarker compounds of thebitumen and the source rock. The result of the oil-source correlation lead to twoconclusions: the one is that the bitumen from Sinian reservoir had a good correlationwith the mudstone from the Qiongzhusi Formation in Sinian, and the mudstone fromthe3rd member of Dengying Formation had a less good correlation with the bitumenfrom Sinian reservoir. The other one is that the mudstone from the DoushantuoFormation and the carbonatite from the Dengying Formation were not the effectivesource of the study area. The comparison on the diversity of the compounds and thecharacteristics of isotope of the gas in the Sinian reservoir among the differentstructures were carried out.According to the result of the comparison, the gas from theSinian-lower Paleozoic stratum of the study area would be divided into the followingthree types. The first type of the gas was the shale gas from the Qiongzhusi Formationin Cambrian in Weiyuan structure; the second type of the gas, sourced from the sourcerock from the Qiongzhusi Formation, contained the gas from the LongwangmiaoFormation in Gaoshiti-Moxi structure, the gas from the Xixiangchi Formation inCambrian and the gas fromthe Ordovician strata. The last type ofthe gas, characterizedby the cracking gas from bitumen, was from the Dengying Formation in Gaoshiti-Moxistructure.For the determining of the charging process and the origin of the fluid from theSinian-lower Paleozoic stratum, the following parameters will be adopted: thecorrelation of the fillings in bores, holes and cracks in the reservoirs in the Sinian-lower Paleozoic stratum among the different structures, the characteristics of fluidinclusions, the characteristics of strontium stable isotope, the characteristics of carbonstable isotope and characteristics of oxygen stable isotope. Based on the result above,a comparison on the diversity of the charging characteristics and the preservecondition of fluid from different structures was carried out. The phenomenon ofcross-formation migration of the fluid was common before the major charging priedof crude oil in the Sinian-lower Paleozoic stratum in the Gaoshiti-Moxi structure,which indicated a poor preserve condition in the Gaoshiti-Moxi structure. However,the phenomenon of the large amount of cross-formation migration of the fluid wasseldom occurred in the Weiyuan-Ziyang structure, which indicated a good preserve condition in the Weiyuan-Ziyang structure. After the period of oil-cracking, thestructures of Gaishiti-Moxi and Weiyuan-Ziyang suffered a process of reconstruction,and structures and the hydrocarbon from them were damaged.Based on the tectonic evolution history and the reconstruction of thehydrocarbon-generation history, the research was focused on the correlation on whenand where the oil and gas generated, transported, accumulated and re-accumulated(ordamaged) among different structures. The correlation on the difference of the formingcondition and pool-forming model of the hydrocarbon accumulation among differentstructures. According to the exposition and erosion during the period of Tongwantectonizationâ†'the formation and evolution of theMianyang-Lezhi-Longchang-Changning intracratonic sagâ†'the evolution of theLeshan-Longnvsi Paleo-upliftâ†'Himalayan orogeny movement, the summarisation ofprimary control factors and accumulation patterns had been concluded. The expositionand erosion during the period of Tongwan tectonization was the primary factor of thedevelopment of the effective reservoirs. The formation and evolution of theintracratonic sag wasthe primary factorofthe development ofthe source-centre and theforming of an effective conducting system. The evolution of the Paleo-uplift was theprimary factor of the evolution of paleo-hydrocarbon accumulation. The Himalayanorogeny movement determined the present distribution ofthe gas and amount ofthe gasaccumulated.