The nearshore sand bar-sheet sedimentary system of the K-Successions sandstones of the Zhujiang Formation (23.8-16.5 Ma) in H21 Area of Huizhou Depression, Pearl River Mouth Basin has been identified in this research according to lithological characteristics, lithofacies, sedimentary sequence and seismic attributes. Seven lithofacies were recognized: fine-grained massive sandstone (Sm), fine-grained trough cross-bedded sandstone (St), fine-grained planar-bedded sandstone (Sp), fine-grained ripple cross-bedded sandstone (Sr), fine grained horizontally-bedded sandstone (Sh), laminated claystone with interbedded siltstone (Fl) and massive mudstone (Fm). Three sedimentary microfacies were further divided: sand bar, sand sheet and interbar. With the progradation and retrogradation process influenced by sea level changing, delta evolution of K-Successions sandstones in H21 area of Huizhou Depression can be divided into four stages: the early stage of the transgressive systems tract, A/S>1; the medium stage of the transgressive systems tract, A/S>1; the end stage of transgressive systems tract; the stage of the highstand systems tract, A/S<1. Different evolution stages controlled the sandstones distribution.
Fan delta systems have long been studied as a kind of good hydrocarbon reservoirs. Although many models have been raised according to their local conditions, there still exists some controversies regarding the formations of the ones developed in continental syn-rift settings. Taking Paleogene Es3 2+3 of Gaobei Slope Belt as an example, this paper conducted a comprehensive geological study and a forward depositional modeling. The results show that the sediment within the study area came mainly from the northeastern Baigezhuang uplift and the northwestern Xinanzhuang uplift over a short distance. The sedimentary system is characterized by subaerial debris flow, subaerial braided channels, subaqueous distributary channels, front sand bars and front sand sheets. Furthermore, the study area is a balance-filled basin that experienced a 3rd order normal regression followed by a transgression during the development of the study interval. The results also show that, a stable subsidence rate with small-magnitude climate changes would be a plausible explain for the entire sequence of the fan delta system in the study area. The tectonic subsidence was indeed the dominant driving force to a continental syn-rift fan delta system, although the climate, sediment supply or lake level might bring some lower order (e.g., 4th and 5th order) superimpositions. This paper proposed a likely model for fan deltas developed in continental syn-rift settings, which is expected to gain some insights into the forming mechanism and provide some supports for the reservoir characterization of similar and correlative sedimentary systems.
In the Z21 oil-gas field, a total of six depositional lithofacies and two depositional elements were identified based on core observation. Three main diagenetic processes, namely mechanical compaction, cementation, and dissolution of Miocene Zhujiang Formation sandstones were identified according to thin section and scanning electron microscope (SEM) of core samples. Cementations mainly contain silica cementation, carbonate cementation, clay minerals and pyrite. A total of three main pore types, residual primary intergranular pores, secondary dissolution pores and micropores, were identified. Sand sheet deposited in low-energy environment and is characterized by relatively low porosity and permeability values. Lager grain-sized sandstones are of higher quality compared to smaller-sized sandstones. Mechanical compaction, calcite cementation and clay mineral cementation play a key role in reducing porosity and permeability, whereas dissolution of feldspar and debris contribute significantly to improving the reservoir quality. The gas charge occurs prior to oil charge, forming a gas cap in the structural high and an oil ring in the lower formation. Irreducible water stored in the lenticular sandstone of low-porosity and permeability reservoir may convert to movable water as the drill and production perform.
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