A large quantity of drilling core, paleontology, geochemistry and geophysics data revealed several features of the Jiyang subbasin during the deposition of the Ek1-Es4 x members:(1) the paleotopography of the gentle slope belt had an extremely low gradient;(2) the paleoclimate frequently alternated between dry and wet periods in a generally arid setting;(3) there was strong weathering around the periphery of the basin;(4) the lake was very shallow;(5) the lake level frequently rose and fell; and(6) the sedimentary environment of the gentle slope belt was an overflooding lake. All of these factors provided favorable geological conditions for the development of an over-flooding lake delta. The lithologies of the continental over-flooding lake delta deposits are complex and diverse. The compositional maturity is moderate to low, and the grain size distribution curves and sedimentary structures indicate the presence of both gravity and traction currents. The sedimentary microfacies associations consist of a combination of ordered superposition of flood channels, distributary channels and sheet sands. The delta exhibits a weak foreset seismic reflection. The over-flooding lake delta deposits are laterally extensive. The sandstone content is high, and the individual sandstone beds are thin. The flood channel and distributary channel deposits exhibit evidence of bifurcation and lateral migration. The distribution of the sandbodies and the oxidation color of the mudstones provide evidence of cyclic deposition. The paleoclimate was the dominant factor controlling the development of the over-flooding lake delta. Due to the frequently alternating wet and dry paleoclimates, the over-flooding lake delta is characterized by the development of a broad upper plain and a lower delta plain. The upper delta plain is characterized by flood channel deposits, whereas the lower delta plain is represented by distributary channel deposits. The transition zone is characterized by the interaction of flood channels and distributary channels
The diagenetic environment, diagenetic responses, diagenetic transformation model and formation mechanisms of high-quality reservoirs(beach-bar sandstones of the Paleogene fourth member) in the Dongying depression were studied through the analysis of fluid inclusions, thin section and burial evolution history. The diagenetic fluids of the beach-bar sandstone reservoirs evolved from early high salinity and weak alkalinity to low salinity and strong acidity, late high salinity and strong alkalinity and late low salinity and acidity, which were accompanied by two stages of oil and gas filling. The fluids at the margins of the sandbodies were continuously highly saline and strongly alkaline. The western(eastern) reservoirs experienced early open(closed), middle open, and late closed diagenetic environments during their burial history. The flow pattern was characterized by upwelling during the majority of the diagenesis(in the east, a non-circulating pattern transitioned into an upwelling current). Due to the evolution of the diagenetic fluids, the diagenetic sequence of the beach-bar reservoirs was as follows: early weak carbonate cementation; feldspar and carbonate cement dissolution and authigenic quartz cementation; late carbonate and anhydrite cementation, authigenic feldspar cementation, and late quartz dissolution; and late carbonate cementation, feldspar dissolution, and authigenic quartz cementation. The diagenetic strength during these stages varied or was absent altogether in different parts of the reservoirs. Due to the closeness of the diagenetic environment and the flow pattern of the diagenetic fluids, the diagenetic products are variably distributed in the sandstones interbedded with mudstones and in the fault blocks. The evolution of multiple alternating alkaline and acidic diagenetic environments controlled the distribution patterns of the reservoir diagenesis and reservoir space, and the reservoir quality index, RQI, increased gradually from the margins to the centers of the sandstones. The closenes
The sedimentary environment,formation conditions,sedimentary characteristics and the basin evolution model of high-frequency oscillatory lake in arid climatic background of the Lower Eocene in the Dongying depression were studied through the analysis of drilling cores,sporepollen,geochemistry and geophysics data.During the sedimentary period of the Eocene Ek1–Es4x formations,because of the frequent alternation between dry and wet climates in the arid climatic background and the gentle paleogeomorphology,the lake level and salinity of the Early Eocene Dongying depression frequently and rapidly increased and decreased,which is referred to as a high-frequency oscillatory lake.The sedimentation and distribution of sediments in this high-frequency oscillatory lake basin were controlled by the frequently alternating dry-wet climates.During periods with relatively wet climate,the seasonal floods and unstable rivers led to the formation of over-flooding lake deltas in the gentle slope belt,and fine-grained clastic sediments,with minor thin layers of gypsum-salt rocks in the sag belt.During the relatively arid climatic periods,sedimentation occurred mainly in the limited area of the sag belt with thick gypsum-salt rocks.Because of the impact of the salinity stratification of the lake water,these gypsum-salt rocks exhibit annular structural features.A sedimentary cycle of the oscillatory lake began with isochronous flood channels and ended with relatively thick gypsum rocks and salt rocks.The thickness of one oscillatory cycle is generally 4–20 m.The superposition of multiple sedimentary cycles of the oscillatory lake constitutes the overall vertical filling sequence of the high-frequency oscillatory lake basin.
