Abstract:

 There are widespread indications of oil and gas in the Chang 64+5 layer of western Mahuangshan, while it proves that there is more water and less oil by well test. Furthermore, the distribution and continuity of sand arent clear. To understand the distribution and evolution of sedimentary microfacies in the area, six kinds of sedimentary microfacies and eight kinds of sedimentary structures were identified and ten kinds of microcontact patterns and five kinds of sedimentary sequence models were summarized by detailed core observation and description and logging data. Each model represents different sedimentary facies.  Reverse cycle is dominant and normal shortterm cycle also exists in the vertical direction; distributary mouth bar and subaqueous distributary channel are developed alternately in the lateral direction. The lower channel sands are “many but small, narrow and thin”, while the upper channel sands are “a few but large, wide and thick”. 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. Its evolution shows three stages: the initial development period (Chang 63), the main facies belts are the predelta and the delta outer front; the earlymiddle development period (Chang 62Chang 61), the main facies belts are the inner delta front and the delta outer front and each microfacies is a single phase and is deposited alternately; the middlelate development period  (Chang 4+52Chang 4+51), the main facies belt is the delta inner front and the subaqueous distributary channel shows a multiphase stacking pattern.

Key words:

Key words: northern area of Western Mahuangshan; sedimentary microfacies; evolution; Chang 64+5 layer