The Study On Sedimentary Characteristics And Sequence Stratigraphy Of Fine-grained Rocks Of The Upper Fourth Member Of Paleogene Shahejie Formation, Dongying Depression

Fine-grained sediments are clay- and silt-sized sediments that have the particle size less than 62 μm. Its deposition and consolidated process are seemingly simple and extraordinarily complex issues. Sedimentology and sequence stratigraphy of fine-grained rocks(FGR) need to be researched deep. To study these questions is beneficial to research the characteristics and distribution of FGR reservoirs that guides the exploration of unconventional hydrocarbon more effectively. Widely distributed FGR with rich unconventional hydrocarbon resources of the upper fourth member of Shahejie formation in Dongying depression are taken as an example to study.Considered the highly important role of organic matter playing in the sediments, diagenetic process and development of reservoirs, with mineral compositions as the primary element, a systematic division method for lacustrine FGR is presented. Firstly, FGR are classified into four groups with silt, clay, and carbonate as the three end-members(threshold level of each indicator, 50%): I, siltstone; II, carbonate; III, claystone; and IV, mixed fine-grained rocks. Then, types II and III are subdivided according to the contents of the three end-members, i.e., TOC(2% and 4%), carbonate(50%), and clay(50%). Taking the sedimentary structures and special mineral compositions into consideration, twelve kinds of lithofacies are identified. A depositional model of FGR controlled by CCD(calcite compensation depth) is proposed.After sedimentary environment analysis in single well, the features between different wells are compared. The climate, water depth and reducibility are characterized by cyclicity and four stages can be identified. The salinity gradually reduced from abnormally high value without cyclicity. The sedimentary environment of FGR periodically varies by four stages. Dongying depression was suffered from four period’s transgression in the second stage.The division method for third- and fourth-order sequence is established with the mainline of lithofacies and multi-method analysis in vertical, compared with sequence stratigraphy of coarse rocks in shallow water. Stratigraphic framework through the basin is presented. The target interval developes a three-order sequence comprising three systems tracts and eight parasequence sets. Passive lake-level rising(TST-1) is discovered as a new type in the TST. A typical FGS parasequence comprises the lower lithofacies indicating deepening water depth and the upper lithofacies indicating shallowing water depth. Pyrite, TOC, potential hydrocarbon generation amount(S1 + S2) and wavelet value undergo increase followed by decrease. Eight types of parasequence are developed. Two categories including the unitary and dual structures are identified. Three origins(relative lake level change, terrestrial input change and transgression) and four sedimentations(biochemical sedimentation, chemical sedimentation, bio-chemi-mechanical sedimentation and mechanical sedimentation) are identified. The developments of high-TOC(>2%) parasequence are mainly controlled by biochemical sedimentation. the developments of low-TOC(<2%) parasequence are mainly dominated by chemical sedimentation. The diversity of FGS parasequences is derived from that of the controlled factors. Five stratigraphic sections are established. Finnaly, stratigraphic framework from the shallow water with coarse rocks to deep water with FGS is presented.Through the study of "point- line- surface", contour map of TOC, carbonate and clay mineral are drawn in the framework of systems tract. Then,evolutionary process of FGR is recovered and researched. The controlled factors of FGR include provenance, climate, physical-chemical conditions of water, tectonism and emergency. Provenance provides clay minerals, carbonate and silts as material basis of FGR after weathering. The periodic change of climate and physical-chemical conditions of water leads to the regularity of FGR in vertical stack. Tectonism controls the distribution characteristics of the FGR through sedimentary facies. It will also be disturbed by emergency(transgression, turbidity, etc.).