Sedimentary Environment And Hydrocarbon Generation Of The Black Shales Series In The Da'anzhai Member And Its Terrestrial Response To The Early Jurassic Oceanic Anoxic Event In The Sichuan Basin,China

The deep time Oceanic anoxic events(OAE)and their terrestrial responses are frontier in current earth system science research.Taking the black shales of the Toarcian(Early Jurassic)Da'anzhai Member in the Sichuan Basin as an example,the systematic analysis of sedimentology,petrology,organic and inorganic geochemistry was carried out by high-resolution dense sampling of typical boreholes.The scale,heterogeneity and driving mechanism of the terrestrial response of Toarcian Oceanic anoxic event(T-OAE)were discussed,and the development characteristics of the source rock,the organic matter accumulation mechanism and the hydrocarbon generation under its influence were analyzed.In addition,considering that the shell-bearing mudstone-limestone combination unique to the Da'anzhai Member is currently a hot area for tight oil exploration in China,the research in this paper is expected to have both important basic science and practical application research significance.Firstly,the petrological characteristics and microfacies types of the Toarcian(Early Jurassic)Da'anzhai Member in Sichuan Basin were characterized,and the paleo-water depth changes were restored,and its response to global sea level rise and fall was revealed.The results show that the Lower Jurassic Da'anzhai Member in the Sichuan Basin is complex in rock types.It can be divided into five types ranging from coarse to fine:fine sand-siltstone,crystalline shell limestone,argillaceous shell limestone,black mudstone containing shell and black mud,which corresponds to five sedimentary microfacies:shallow lake sand dam,high energy shell beach,low energy shell beach,shallow lake mud and semi-deep lake mud,which reconstructed the water depth of the Early Jurassic Sichuan Basin,which is related to the global sea level.Plane changes have temporal consistency over long periods and are generally in the period of retreat after the early Jurassic global transgression(high sea level).In the context of this lake retreat,it also experienced three complete sub-cycle of lacustrine transgression-lacustrine regressive.These characteristics reflect the possible occurrence of terrestrial response to T-OAE in the black rock series of the Da'anzhai Member of the Sichuan Basin.Further comparison of the carbon isotope geochemical profiles of typical boreholes reveals that the Sichuan Basin did have a terrestrial response to the T-OAE during the Toarcian period,and it was first discovered that this response occurred on the basin scale.However,it is heterogeneous and there are differences in depositional environments between different regions.The mechanism of terrestrial response to the T-OAE is unlikely to be transgression,but is more likely to be caused by climate disturbances in large volcanic provinces and then induced by atmospheric-hydrological cycles.Secondly,it is clarified that the high-quality source rocks in the Sichuan Basin could developed during the period of T-OAE.The lacustrine shell-containing limestone-mudstone combination source rocks in the Da' anzhai Member are generally moderate to very good source rocks(TOC=0.5-2.6%,average 1.0%),and the bio-precursor includes higher plants and algae originated amorphous organic matter,The organic matter type ? to ?,the degree of thermal evolution is mainly mature,with a small amount is highly mature(VRo?1.0?1.5%).For different lithologies,shale,shell mudstone and a small part of the shell limestone which were influenced by T-OAE have higher abundance of organic matter(TOC>1.0%),better organic matter type(?1-?2 type),and more potential for oil generation;In contrast,most of the shell limestone and silty mudstones which were not influenced by T-OAE have low organic matter abundance(<1.0%),organic matter type is humic,and the hydrocarbon potential is limited.The unconventional oil and gas resources potential of the Da'anzhai Member of the Sichuan Basin under the influence of T-OAE was evaluated.The results show that the shale or shell limestone layer with TOC>2.0%in the Da'anzhai Member has good potential for hydrocarbon generation and drainage,and it is easy to be adjacent to the shell limestone and shell mudstone layer rich in microporosity and high content of brittle minerals.The formation of unconventional oil and gas resources,TOC=0.7?2.0%of shell limestone and shell mudstone pores and brittle minerals have potential unconventional oil and gas potential.Thirdly,it is clarified that the accumulation of organic matter in the Da'anzhai Member of the Sichuan Basin under the influence of T-OAE is mainly controlled by high productivity.The source rocks of the Da'anzhai Member are deposited in a high-productivity,metamorphic sedimentary environment.The accumulation of organic matter is controlled by high productivity,sedimentary environment and sedimentation rate.The productivity factor is the main factor,followed by the sedimentary environment,and the deposition rate is the smallest.The control of high-quality source rock distribution by T-OAE is mainly carried out through high productivity.The warm and humid paleoclimate caused enhanced weathering,brought nutrients to the sedimentary water body,and improved productivity..Finally,hydrocarbon generation characteristics was analyzed and the hydrocarbon expulsion mode of the Da'anzhai Member was established by thermal simulation experiments.The result shows that the oil-generating rate of the source rock increases rapidly after entering the oil-generation threshold.The total oil generating peak is reached 311 kg/tc at Ro=1.15%,while the oil produced is mainly occurred as residual oil.At the thermal evolution of Ro=1.26%,the expulsed oil exceeded the residual,and the total oil yield began to decrease.The major source of natural gas changed from kerogen cracking gas to oil cracking gas.The peak of gas and total hydrocarbon generation rate were reached at Ro=1.46%,which were 468.05 kg/tc and 519.53 kg/tc,respectively.Generally speaking,the hydrocarbon source rock of Da'anzhai Member has good hydrocarbon potential,and has potential for both oil and gas generation.The oil generation potential is weaker than that of alkali lake phase source rock(with type I kerogen),and is stronger than the freshwater lake phase source rock(with type ?2 kerogen).However,due to the depth of the Da'anzhai Member of the study area between 1500 and 3000 m,In fact,the source rock is actually dominated by oil generation.This research shows that the black shale series of the Da'anzhai Member in the Sichuan Basin provides us a rare example for studying the terrestrial responses to the OAEs.The deep time Oceanic Anoxic Events do have terrestrial responses and are earth system events.But the scale,range and mechanism of the response,and the impact on organic matter accumulation and hydrocarbon generation are far more complicated than in the past,and further research is needed.The Lower Jurassic Da'anzhai Member in the Sichuan Basin is an important target worthy of attention in the future exploration for both conventional and unconventional oil and gas.