Sequence Stratigraphy, Fluvial Reservoirs, and Subtle Oil Bearing Traps

Abstract The study area is located at Reo area, Liaohe basin, northeast China. This study documents the types and distribution of hydrocarbon reservoirs and oil-bearing traps associated with the main depositional environment of fluvial deposits interbedded with volcanic rocks accumulations, especially in the target Formation of Sand River Street 3 (Here called SRS3). Petroleum exploration in the study area began from 1970's. Early exploration targets were structural traps located along anticline structure belt formed by volcanic rocks accumulations, and many drilling wells along anticline structural belt were failed because the sand reservoirs were poorly developed. The subtle oil bearing traps were not found before ideas of sequence stratigraphy were applied to the study of reservoir prediction. To make the reservoirs distribution clear was one of the greatest objective within the area at this time of the study, this paper was aimed to identify the paleoenvironmental factors which affect the distribution of reservoirs, and predict the spatial and temporal distribution of reservoir facies in areas away from well control. The study will aid in the recognition of subtle oil bearing traps. The data in this study were collected through detailed stratigraphical, sedimentological and paleobiological analyses of 23 well loggings, 31 cored intervals with microfossils in SRS3 Formation and 3-D seismic data of this area. Based on these data, a general sequence stratigraphic framework was reconstructed, and the channel reservoirs were predicted through stratigraphy correlation and JASON reversion software. Based on the cored intervals, microfossils, well loggings and seismic reflectors, a sequence stratigraphic framework was reconstructed to correlate the genetic units and determine the spatial distribution of the oil bearing sand bodies. Stratigraphic terminology presented in this paper is adapted from Van Wagoner et al. stratigraphic interpretations led to the identification of two sequences (third order): Seq1 and Seq2 within the SRS3 Formation, and Seq2 was selected as the focus of this study. 1. The transgressive system tract (TST) displays backstepping stacking pattern. It is bounded below by the fluvial entrenchment surface landward and first transgressive ravinement surface basinward. The maximum flooding surface or condensed section forms the upper boundary. The highstand system tract (HST) is characterized by progradational facies successions and off-stepping stacking pattern. It is bounded below by the maximum flooding surface and above by the fluvial entrenchment or regressive ravinement surface. 2. Seq1 deposited in a river-dominated delta environment. Seq1 can be subdivided into three parts according to the relative lake level change: transgressive system tract (TST1), condensed section (C.S), and highstand system tract (HST1). TST1 deposits consist of thinning-upward sandstone interbedded with delta front mudstones, whereas HST deposits consist of thick dark gray prodelta mudstones containing large amounts of ostracod fossils and coarsening-upward sandstones which interbedded fan delta front mudstones and fan delta plain coal seams. It should be noted that thick volcanic rocks were developed during the HST1 early deposits of delta front mudstones. The volcanic rock accumulation is approximately 30 meters high.