Study On Depositional System Of Changxing Formation Reef Beach In Wu Baiti Block,Sichuan Basin

The Kaijiang-Liangping Trough is located in the northeastern part of the Sichuan Basin and is NW-SE oriented,Since Kaijiang and Liangping are connected to the northwest by Daxian County,Pengchang and the Guangwang Trough,In general,it is a carbonate platform and a reef-beach facies belt.The Permian Changxing Formation is one of the major oil and gas producing layers in the Sichuan Basin.In the past 30 years,many experts and scholars have participated in the study of the Changxing Formation reef-beach reservoirs in the Kaijiang-Liangping Trough,and found a series of large and medium-sized gas fields such as Longgang,Yuanba and Puguang.Wu Baiti block is located just at the edge of the southeastern end of the Kaijiang-Liangping trough,Based on the current exploration status of other blocks around the trough,The Changxing Formation of the Wu Baiti block should have the conditions for the development of extensive biogenic reef reservoirs.However,from the current survey results of the five hundred ladders collected,it is obviously not as expected.Through the comparative analysis of the existing wells in the study area,it is found that the main reason for the fact that the real reef reservoirs are actually encountered in the exploration is that the lack of effective seismic identification methods for the reef reservoirs and the main controlling factors for the development of the reefs are not clear,Therefore,the research on the sedimentary environment,spatial distribution and reservoir characteristics of the biogenic reef reservoirs in the study area is not deep enough.To this end,this paper studies the depositional system of the Changxing Formation reef-beach in the Wu Baiti block of the Sichuan Basin.To this end,this paper studies the reef-strait favorable zone of the Changxing Formation reef-shoal in the Wu Baiti block of the Sichuan Basin.In order to improve the success rate of late exploration.Research on depositional systems,Mainly using the predecessors' research results,drilling,logging and 3D seismic data in the study area,Firstly,using the standardization theory of sequence stratigraphy,the sequence stratigraphic characteristics of the Changxing Formation single well and the connected well were analyzed.Secondly,seismic simulation analysis is carried out,and the correspondence between formation lithology and seismic reflection is established and applied to the no drilling area.At the same time,combined with the traditional seismic facies analysis and the biogenic reef over-soil seismic profile,the seismic identification model of the Changxing Formation reef-beach in the study area is summarized,and the large scale depositional environment of the reef-beach is divided and the paleo-geomorphological characteristics of the Changxing Formation in the study area are restored.On this basis,using the subdivision seismic facies analysis method and the 3D seismic data of the study area,The seismic scanning interpretation technique is applied and the root mean square amplitude stratigraphic slice analysis is used to identify the skeletal facies,and the relationship between the sedimentary structures(sand dams,reefs,beaches,etc.)and the water flow,and the gully connections are used to analyze the water flow path.Finally,based on the method of “skeletal facies + major environment + water flow path = depositional system”,the reef-beach depositional system of Changxing Formation in the study area was reconstructed.Through the above research work,the following aspects have been recognized:(1)This paper divides the Changxing Formation of the study area into two third-order sequences(SQ1,SQ2).The two third-order sequences are further divided into five sedimentary system domains(from bottom to top,LST,TST,HST,FSST,LST),equivalent to five 4th-order or Intermediate periodic depositional cycles.(2)The reefs of the Wu Baiti block developed on the gentle slopes zone.The seismic identification features of the platform margin reefs in the study area were summarized by the method of horizon bottoming and seismic scanning interpretation: location,shape,internal structure,and attributes.Marking and overhanging structures of some parts of the overlying stratum.(3)The reef-beach depositional system and the evaporative tidal flat depositional system are two depositional systems developed under different sea level conditions.During the relative sea level rise and the high rise slowly,A reef-beach depositional system usually developed with a combination of “Trough-slopes-reefs-lagoonsbeaches-open platform”;while in the relative sea level decline and low period,The evaporative tidal flat depositional system usually developed with a combination of “trench-slope-lagoon-evaporation tidal flat-muping”.(4)The development of reef cover reservoirs requires three basic conditions,the isolated reefs of the platform margin,the high energy beach and the evaporative tidal flat.The isolated geomorphology of the isolated reef can provide a basic geological condition for the development of high-energy beaches,evaporative tidal flats,and few muddy entry;high-energy beaches can develop porous carbonate rocks with primary pores;evaporative tidal flats It can provide a brine environment where dolomitization occurs.(5)Based on the depositional system map of the Changxing Formation in the study area and the development model and control factors of the reef-beach reservoir,Combined with the top root square root amplitude slice of Changxing Formation,the distribution range of favorable zone in the study area is delineated.