Characteristics and Depositional Regularities of Oil Shale in Upper Cretaceous of Southern Songliao Basin
|Course||Mineral prospecting and exploration|
|Keywords||Songliao Basin Upper Cretaceous Oil shale Depositional environment Sequence stratigraphy Metallogenic regularity|
Songliao basin is located in Northeast China, covers the area of E119°40′～E128°24′, N42°25′～N49°23′and stretches in NNE direction, with length of750km, width of 350km and total area of 260,000 km2. It’s a Mesozoic-Cenozoicbasin. It is divided into the south and north by Songhua River. The south ofSongliao basin mainly located Jilin province, with total area of 133,000 km2.The research of Songliao basin is mainly on conventional oil and gas, but oilshale resource in Songliao basin is abundance, and it is an importantcomplementarity energy resource of oil and gas. Currently, it is emphasizeparticularly on the classification of oil shale and its exploitation. There is a lack ofstudying on the cause of formation and its depositional environment. My thesis notonly has some help to develop the theories of oil shale’s cause of formation, butalso has a huge realistic meaning.Oil shale in the south of Songliao Basin deposited in the 1st member ofQingshankou formation and the 1st -2nd member of Nenjiang formation. It’sdistributed almost of the south Songliao basin. The oil shale in Qingshankouformation deposited in the lower of 1st member of Qingshankou formation, thereare 5 layers deposited in the southeast of the basin. The oil shale in Nenjiangformation deposited in the middle of 1st and the lower of 2nd member of Nenjiangformation. There are 3 layers deposited in the basin separately in the 1st and 2ndmember of Nenjiang formation. The thickness of oil shale in Qingshankouformation is more than in Nenjiang formation. But the acreage of oil shaledeposited in Nenjiang formation is bigger than in Qingshankou formation. Thethickest of oil shale exceeded 40m in Qingshankou formation and 10m inNenjiang formation.Oil shale in southern Songliao Basin are gray-taupe-black in colour, streakgray, conchoidal fracture, pelitic texture, compact-massive and parallel beddingstructure, with middle -grade shale oil , commonly about 5%, the highest about12.10%. There are some apparent positive correlation between oil grade and theircalorigic value, the percentages of C、H、O、N respectively, and negativecorrelation between oil grade and ash content. The organic matter of oil shale is oftype I—II. The vitrinite reflectance of oil shale in southern Songliao Basin iscommonly about 0.5—1.3%, revealing the early stage of oil generation, inNong’an area is commonly lower than 0.5%, indicating the immature stage.Oil shale was developed in humid-warm and tropic-subtropical climatic.Abundant plants grew in such climatic condition, with plenty of organic matter,and it’s easy to hold palaeo-salinity and dept, which are propitious to the formationof oil shale.From Albian （K1） to Maastrichtian （K2）, the global craton had experiencedsix-time large sea-level change periods. Songliao basin had experienced five largelake-level change cycles from Shahezi formation to Nenjiang formation, and thedevelopment of Songliao basin had come to golden ages from Quantou period toNenjiang period. During that time, the rising extend of lake-level was the biggest,form Quantou period to Qingshankou period was the fourth transgression stage ofSongliao basin, from Yaojia period to Nenjiang period was the fifth transgressionstage of Songliao basin, and which was also a super size transgression cycle. Thetwo-time lake-level rise had direct relation to the global sea-level rise, which isone of the most important factors that cause the two-time transgression ofSongliao basin, and The two-time lake-level rising respectively corresponded toAlbian and Santonian-Campanian of the global sea-level change. The oil-shaledeposited in the 1st member of Qingshankou formation and the 1st and 2nd memberof Nenjiang formation had nearly relation to the maximum transgression, all of theoil-shale developed in the maximum transgression stage, which indicated that theoil-shale was the result of the two-time maximum transgression stage of the basin.The sedimentary environment in the early period of Qingshankou formation andNenjing formation belonged to neither single continent nor genuine marine, but acontinental-marine mixed environment. The sea transgression divided the lake intoupper bed and lower bed, and made the lower water lack of oxygen, theoxygen-deficient environment make the organic kept well. The oil-shale wasdeposited in anoxic environment.The sedimentary facies sequence in Qingshankou formation is inherited fromQuantou formation. It is transitional from shore-shallow lake in the fourth memberof the Quantou formation to deep lake-shore-shallow lake-regression delta inQingshankou formation and it is developed from shallow lake to deep lake inQuantou formation in the center of the basin. The distribution of Nenjiang stratumexceeds greatly the scope of the recent basin, especially the 1st and 2nd member ofNenjiang formation distribute most extensively than ever. As sedimentarylithofacies sequence, a whole 2nd order cycle deposited in Yaojia formation andNenjiang formation and it is transitional from shore-shallow lake in upper Yaojiaformation to semi-deep-deep lake in the 1st and 2nd member of the Nenjiangformation. A whole cycle including transgression and regression deposited fromYaojia formation to Nenjiang formation. It is deposited deep lake in the center ofthe basin and shore-shallow lake around when the 1st and 2nd member of theNenjiang formation deposited, there is no other facies, the characteristic is mostobvious in the 2nd member of Nenjiang formation. It is the third supertransgressive stage.in Songliao basin. Oil shale deposited mostly in semi-deep-deep lake.The sequence when oil shale deposited is also mainly subject to the influenceof climate, sea-level change, lake-marine lingking events and so on. Songliaobasin is a typical large scale depression lacustrine basin with large depositionalarea in the stage of Qingshankou–Nenjiang formation. Base on the research ofoutcrop, well logs, seismic and drilling data, 8 local unconformity surfaces（SB11～SB18） and 7 third order sequences are identified from it in the center-west of thebasin, that is sequence XI～XVII. Three sequences deposited in Qingshankouformation, three in Nenjiang formation, they were Sequence XI、XII、XIII andSequence XV、XVI、XVII. The oil shale deposited in 1st Qingshankou formationand 2nd Nenjiang formation deposited in the lower part and upper part of thedepression sequence, it’s the sequence XI and sequence XV. Oil shale is condensedsection deposition, which is a reliable direct recognizable mark of parasequenceboundaries.According to the research of the oil shale’s sedimentary character in Songliaobasin, we established the oil shale’s depositional model of the Songliao basin atthe base of the four-division scheme within a third order sequence （Liuzhaojun,2002）.The oil shale mainly developed in the transgressive systems tract（TST） andhighstand systems tract（HST）.The accommodation increase rate in basin is morerapid than sediment supply increase rate in these two periods, so the basin is instarved deposition, with thin grain size of sedimentary, such as dark mud stone andoil shale, which named as condensed section in continental sequence stratigraphy,and the condensed sections is not only one layer in the whole lake floodingprocess. The oil shale deposition begins with the lake flooding cycle, and itsbottom boundary is the each start of the lake flooding surface.