Origin And Source Of Sinian Bitumen In The Central Sichuan Basin

Bitumens are widely developed in the Sinian strata of Sichuan Basin and its neighboring areas.As we all known,bitumen is an intermediate product between petroleum and natural gas.It records various geological and geochemical processes petroleum underwent.Geological information inferred from bitumen will aid to understand the processes of hydrocarbon accumulation in the Sinian strata,which will play an important role in hydrocarbon prediction and natural gas exploration in Sichuan Basin.This paper chooses the Sinian Dengying Formation bitumen in the central Sichuan Basin as the study object.The occurrence characteristics of bitumen are studied using optical microscope and scanning electron microscope observations.There are two types of bitumen in the Sinian strata in the central Sichuan Basin.The first is protobitumen,which is formed through the thermal degradation of in-situ organic matter in source rocks.The second is solid bitumen,which can be subdivided into source rock solid bitumen and reservoir soild bitumen according to occurrence location and migration distance.Sinian protobitumen and source rock solid bitumen mainly occur in Dengying 3rd member black argillaceous dolomites,indicating that argillaceous dolomites had abilities of hydrocarbon generation and underwent the processes of hydrocarbon generation.Sinian reservoir solid bitumen,however,widely distributed in Dengying dolomites,implying that widespread occurrence of hydrocarbon charging processes in the geological times.Sinian reservoir solid bitumens have higher random reflectance values?average 3.40 %?and bireflectance values?4.2 %⁶.9 %?,which exhibit a strong optical anisotropy and various coke structures.The coke structures include fine-grained,medium-grained,coarse-grained mosaic coke structures as well as flow-like to domain coke structures.The types of coke structures in reservoir solid bitumens directly depend on the nature of bitumen precursors.Solid bitumens with fine-grained to coarse-grained mosaic coke structures evolved from asphaltene-rich heavy oil precursors,whereas solid bitumens with flow-like to domain coke structures originated from deasphalted heavy oil precursors.The formation of these heavy oils and reservoir solid bitumens is mainly controlled by thermal alterations,including normal thermal maturation and abnormal hydrothermal alteration.Since the adsorption of asphaltenes onto authigenic kaolinites plays a role in the precipitation of reservoir solid bitumens,natural deasphalting is also a formation mechanism for Sinian reservoir solid bitumens.In addition,biodegradation of Sinian reservoir solid bitumens might be not occurred in the study area.Rare earth element?REE? compositions of Sinian-Cambrian reservoir solid bitumens in the central Sichuan Basin and Weiyuan area were analyzed.Although The ?REE values of Sinian-Cambrian reservoir solid bitumens are very low?0.20 ?g/g to 2.36 ?g/g?,reservoir solid bitumens from different geographic locations,strata as well as occurring spaces can be distinguished by using REE and their associated parameters.Thus,REE could be used in solid bitumen classification.Factor analysis and principal component analysis of REE data showed that REE concentrations played a dominant role in solid bitumen classification,and fractionation degree between light and heavy REE,such as LREE/HREE and La_N/Yb_N ratios,played a secondary role in solid bitumen classification.Two geological processes together controlled REE concentration and pattern in reservoir solid bitumens,including inheritance from source rocks and water-rock interaction.A perfect quadratic function between ?REE values and ?¹³C values of reservoir solid bitumens indicates the major factor controlling REE concentrations in reservoir solid bitumens is the type of organic matter of source rocks.The ?REE values and ?¹³C values of Sinian-Cambrian reservoir solid bitumens in the central Sichuan Basin and Weiyuan area are very low,which have good comparison with prior studies of reservoir solid bitumens that originated from the lower Cambrian shales.Thus,Sinian-Cambrian reservoir solid bitumens in the central Sichuan Basin and Weiyuan area might be also sourced from the lower Cambrian shales.On the other hand,water-rock interaction resulted in formation of authigenic minerals in vesicles of reservoir solid bitumens,such as illite,barite and etc,but these minerals appeared to have little influences on REE composition of reservoir solid bitumens because of their small proportions.However,these minerals have great influences on REE composition of reservoir solid bitumens in stylolites.Core observations indicate that Sinian-Cambrian potential source rocks in the central Sichuan Basin include the lower Cambrian black shales and polymetallic Ni-Mo ore layer,Dengying 3rd member black argillaceous dolomites,as well as Dengying algal dolomites and dolomites.Petrology and geochemical studies of these source rocks imply that the organic matter is mainly type I-II,and has higher thermal evolution degree.Dengying algal dolomites and dolomites are deposited under oxic conditions,which have lower values of organic matter abundance,belonging to very poor to poor source rocks.Dengying 3rd member black argillaceous dolomites can be divided into two portions.The lower portion is rich in organic matter,which is deposited under suboxic conditions,belonging to fair to good source rocks.However,the upper portion is depleted in organic matter,which is deposited under oxic conditions.It is considered as poor source rocks.Qiongzhusi polymetallic ore layer is deposited under sulfidic conditions,which has the highest organic matter abundance,belonging to excellent source rocks.However,due to its thin thickness,the generation potential is limited.Qiongzhusi black shales have high abundances of organic matter,which are deposited under the persistently anoxic conditions.They are widely developed in the Sichuan Basin and its neighboring areas,which is evaluated as the most realistic and potential good source rocks.Bitumen-source rock correlation is made by using carbon isotopes and trace elements?e.g.,V/Ni ratio?,indicating that Cambrian-Sinian reservoir solid bitumens in the central Sichuan Basin and Sinian reservoir solid bitumens in Weiyuan area have a common source.They were all sourced from the lower Cambrian Qiongzhusi black shales deposited under anoxic conditions.Although Dengying 3rd member argillaceous dolomites and Qiongzhusi polymetallic ore layer have hydrocarbon potentials,the contributions of these source rocks to hydrocarbons in Sinian strata might be small.