Seismic-Sedimentary Facies Study Of Penglaizhen Formation In Luodai Gas Field

The Luodai Gas Field is located in the Longquanyi District of Chengdu City and is structurally located on the east slope of the middle section of the western Sichuan Depression—the west side of the Longquan Mountain North-East Tectonic Belt.According to previous research results,it can be known that the Luodai gas field is a multi-layered and multi-type compound gas field with an anticline structure background.The structural characteristics and the distribution of sand layers are the main factors affecting the distribution of gas reservoirs.The Penglaizhen Formation gas reservoir is one of the main gas reservoirs in the Luodai Gas Field because of its shallow depth and large thickness.The gas production capacity of Luodai Gas Field has important influence on the development of industry and agriculture and the life of residents in major surrounding towns.However,at present,the gas field is in a stage of diminishing production.In order to maintain stable production of the gas field,it will not affect the development of the surrounding cities.It is imperative to clarify the structural features of the gas field and the distribution of high-quality sands,so as to provide basis for the deployment of development wells.After clarifying the research status and problems of the study area,a large number of relevant data such as geology,well logging,and earthquakes were collected.On the seismic section,identify the signature layer of the top and bottom sections of the Peng 4 section of the study area,trace and explain the top and bottom sections of the Peng 4 section according to the marker layer,and compile the top and bottom structural diagrams of Peng 4 section to understand the structural characteristics of Peng 4 section.By observing the cores of a large number of wells in Penglaizhen Formation,and analyzing their lithological features and sedimentary structures,it is clear that the Penglaizhen Formation in the study area develops a set of shallow water delta sedimentary systems.The main sedimentary microfacies include underwater distributary channels,underwater distributary interchannel channels,crevasse fans,estuary sand dams,remote sand dams,mat sands,and natural embankments.Through well logging curve calibration and single-well logging phase division,the logging response characteristics of various types of sedimentary microfacies have been clarified:underwater distributary channels are medium-high amplitude box-shaped,bell-shaped,and the estuary sand dam is a medium-high funnel shape.Burst fans are medium-sized funnel-shaped and medium-low-geared.Far-bars dams are medium-low-slot funnels.Mat sands are low-medium-toothed,finger-shaped.Natural embankments are low-width funnel-shaped,tooth-shaped.The underwater distributary channel bay is a micro-dentate-toothed line.Establish a comparative sand grid for the control wells to determine the vertical and horizontal evolution characteristics of the sedimentary facies.The seismic cross section was calibrated by GR logging curves to identify the seismic response characteristics of various sedimentary microfacies:the underwater distributary channel was of medium-strong amplitude,and the underwater distributary channel bay was very weakly amplitude or non-reflective.The estuary dam is of poor continuity with moderate-weak amplitude,the bursting fan is of short-shaft moderate-weak amplitude,and the mat sand,natural embankment,and far sand dam have weak amplitudes of varying degrees.In the course of calibrating the seismic profile,it is recognized that there is a close relationship between seismic amplitude attributes and various types of microfacies,and,there is a positive correlation between sand body thickness and seismic amplitude.Based on this finding,the Jp₁² and Jp₁³ sand groups were traced and interpreted in the study area,using the regional signature layer at the top and bottom of the Peng 4 section,and the amplitude attribute map was extracted as the main reference factor for the sedimentary microfacies characterization.Finally,based on the core,facies,wellbore,and seismic facies research results,a comprehensive compilation of sedimentary facies maps is provided to provide the basis for the development of next-step development wells in the study area.Through structural interpretation,it is recognized that the Peng 4 section of the target stratum is an anticline structure,the anticline axis is northeast-southwest,the elevation is highest in the northeastern corner,and the elevation is lowest in the southeastern and western margins,and the elevation difference in the same sand group is 110-120m.There are no large faults in the area and only scattered small faults develop.The Jp₁² and Jp₁³ sand formations of the Luodai Gas Field mainly developed submarine distributary channels and submarine distributary channel intertidal microfacies;two underwater distributary channels entered the study area from the northeast,and the rivers converged and separated.A mesh-like river is formed throughout the region;on both sides of the subaqueous distributary channel,a tying fan and a mat-like sand microfacies are developed.The microfacies of the estuary sand dam is developed locally,and the microfacies of the natural embankment and the remote sand dam are rarely developed.As a whole,the two sets of sand formations are all developed in the northern part of the underwater distributary channel.The southern channel is narrower and thinner,while the underwater distributary channel in the east is obviously more sparse than the western one.In terms of the distribution area of the underwater distributary channel,the Jp₁³ layer is slightly larger than the Jp₁² layer,which is presumed to be the reason why the lake surface rises slowly and the channel sand body retreats.