Characteristics and Evolution of Sedimentary Microfacies of Chang 6–4+5 Layer in the Northern Area of Western Mahuang Mountain

Although there are widespread indications of oil and gas in the Chang 6–4+5 layer of western Mahuang Mountian, well test proves that there is more water and less oil with the distribution and continuity of sand being not yet clear. To understand the distribution and evolution of sedimentary microfacies in the area, six kinds of sedimentary microfacies and ten kinds of sedimentary structures were identified, and ten patterns of contact between microfacies and five kinds of sedimentary sequence model were summarized through detailed observation and description of the core of rock and logging data. Each model represents different sedimentary facies. Reversed cycle is dominant and normal short-term cycle also exists in the vertical direction; Mouth bar and subaqueous distributary channel developed alternately in the lateral direction. The lower channel sand is “large in quantity but small in size with the layer narrow and thin”, whereas the upper sand is “few in quantity but large in size with a wide and thick layer”. The “single-factor analysis and multifactor comprehensive” research method was used to divide this area into three phase belts—delta inner front, delta outer front, and predelta. Subaqueous distributary channel, distributary mouth bar, and interdistributary deposits are prevailing in the delta inner front. Distal bar and sand sheet are dominant in the delta outer front, and predelta mud is dominant in the predelta. According to modern sedimentary facies model, the area is a constructive braided river-lake delta. Three stages appeared in its evolution: the initial development period (Chang 63), in which the predelta and the delta outer front are the main facies belts; the early-middle development period (Chang 62–Chang 61), in which the inner delta front and the delta outer front dominates and each micro-facies is a single phase and is deposited alternately; the middle-late development period (Chang 4 +52–Chang 4 +51), in which the delta inner front dominates and subaqueous distributary channel shows multiphase stacking pattern.