Sedimentary Environment Of The Middle-lower Jurassic Lacustrine Carbonates In The NE Region Of The Central Sichuan Basin

Based on detailed field work, core, sections and geochemistry etc, we havestudied the lithology, sedimentary environment and facies of the lacustrine carbonatemid-to-lower Jurassic (including Dongyuemiao, Ma′anshan, Da′anzai andLianggaoshan) in the central Sichuan basin. It is very helpful to understand on thedynamic features of climate during early to middle Jurassic times, and the explorationprospects of oil and/or gas in the central Sichuan basin.The sign of sedimentary facies includes primary color, structure and texture,paleontology and logging etc. Thus, the lower-to-middle Jurassic sequence wasdominantly deposited in shore lake, shallow lake and semi-deep lake. There iswidespread ostracodes, bivalve, pleopod and lamellibranch etc. in the sequence, aswell as the sedimentary structure and texture including wormhole, slump structure andstorm rock etc. Furthermore, the Da′anzai strata was deposited in an open freshwaterlake indicated by the13C,18O isotopes.Compared with the13C,18O isotopes and the change of Fe/Mn, Sr/Ba, Rb/Sr, itsuggests that there is a significant change in the sedimentary environment of themid-to-lower Jurassic sequence. It is characterized with shallow lake, warm climateand clear water with the deposit of Dongzuemiao strata, such environment favors forthe organism growth. However, the Ma′anshan strata was deposited in suchenvironment: warmer climate, high magnitude of rainfall, thus mid-high depth andturbidity water, which is not suitable for the deposit of carbonate.The Da′anzai strata is composed of three members as: Dashan, Dayisan and Dayimembers. The Dayisan member was deposited in the micro-facies of shallow lakemud and shell bank. The first micro-facies is dominant with lower temperature(relative to the Ma′anshan period), stable lake level, smaller content of oxygen, thuswith a reducing environment. However, the microfacies of shell bank has much warmer climate, frequently fluctuating and lower lake level, thus a higher content ofoxygen suitable for the organism growth. Furthermore, it is characterized with ahigher lake level (relative to the Dayisan period), warmer weather with the deposit ofthe Dayi member, which is suitable for the organism growth. In the early Dayi period,there was dominant with fluctuating and turbidity water possibly due to strong wind,the lower section of the Dayi member was deposited in the facies of lakeshore.However, it was characterized with shell limestone and mud band predominantlydeposited in the facies of shell bank in the upper section of the Dayi member. This isprobably resulted from frequent fluctuation of lake level. Due to the influence ofterrestrial clast and shrinking lake, the content of carbonate in the upper section ofLianggaoshan member decreased (relative to the Da′anzai), thus with much lessorganism.Therefore, it suggests that the Da′anzai strata is the best strata of explorationprospects in the central Sichuan basin, combined with physical features of reservoirsin region, e.g. porosity and permeability.