The Evolution Of The Ancient Lake And Tight Oil Reservoirs Formation Of The Early And Middle Jurassic In The NE Part Of Central Sichuan Basin

Based on the drilling core and outcrop observation and petrography, this paperanalyzed Carbon and oxygen isotopic, Major and Trace element geochemistry,expounding the relationship between the ancient lake evolution and tight oilreservoir formation of early and middle Jurassic in the NE part of Central SichuanBasin. It is very helpful to understand on the dynamic features of ancientenvironment and climate during early to middle Jurassic times, and the explorationprospects of tight oil in the central Sichuan basin.The main rock types of the tight sandstones are lithic sandstones and feldsparlithic sandstones, rich in lithic and poor in feldspar. The tight carbonate rocks aremainly micritic shelly limestones and the containing shelly micritic limestones.Reservoir densification is caused by compaction.Chlorite ring-side cementation has aprotective effect on the original intergranular pore, effectively improved the reservoirphysical properties. The secondary pores and micro-cracks play an important role inthe reservoir space. The dissolution is the main constructive diagenesis.The climate and paleoenvironmet of the ancient lake determine depositioncombination of phase sequence and sand body and shell beach body developmentcharacteristics in the study area. According to the deposition characteristics, theLonggang area is in a relatively quiet depositional environment, but also has a stablesupply of land-based sources, forming typical lacustrine hybrid sediments, whichcontain restricted hybrid sedimentary rock and generalized hybrid sedimentarysequence. When hybrid deposition appears in Semi-deep lacustrine facies orsurrounding area, nearly source rocks, is conducive to the formation of tight oilreservoirs. Sr concentration and Sr/Ba ratio indicating the ancient lake is an inlandfreshwater lake and absence of marine transgression during the Daanzhai Period.The paleoclimate index,∑（(Fe+Mn+Cr+Ni+V+Co)/∑(Ca+Mg+Sr+Ba+K+Na)ratio indicate semi-moist conditions during the early and middle Jurassic,Overall, the climate condition is stable, but the climate has a tendency to getting dryfrom early Jurassic to middle Jurassic. Paleoredox ratios, including Ni/Co, V/Cr, andTh/U ratios infer dysoxic to oxic conditions for Daanzhai and Dongyuemiao, oxicconditions for Maanshan and Lianggaoshan.Compared with facies cycles and Fe/Mn, Sr/Ba ratio,it suggests thatDayisan sub-member was lake transgression period, and high-water period stay for alonger period of time, lacustrine basin was stability, and most favorable forhydrocarbon source rocks formation. Lake level changed frequently during Dayiperiod. Deposit the multiple sets of sandstone, shell limestone and mudstone,conducive to the formation of tight oil reservoir. Tempestites can be observed thedrilling core, which positions were consistent with the maximum flooding surface.The storm rocks and non-storm deposits appear alternately, in the section, which isalways surrounded by non-storm deposits. This special tight oil reservoirs, whichtransformed by storm,go deep into lake-basin center, with mixed geneticcharacteristics,can open a new frontier for the search for tight oil and gas reservoirsin the Sichuan Basin and elsewhere.Combining typical drilling instances of the study area, I believe that theDayisan’s middle and late period is very conducive to tight oil reservoirformation,due to paleoenvironmental and paleoclimatic changed frequently, andhave a stable terrigenous clastic supply. Therefore, Longgang area can be as a tightoil development pilot area, and should focus on the middle and upper section of Dayisub-member.