Comprehensive Geologic Study On Majiagou Formation Gas Reservoir Of Lower Paleozoic In Fuxian Area Of Yan'an Gasfield

In recent years,tight carbonatite gas reservoir of Majiagou Formation of lower Paleozoic has became issue of exploration research.Yan'an gasfield has good gas production in western exploration area,but there are still problems that the exploration area has unclear reservoir characteristics and insufficient awareness of gas reservoir control factors in southeast of Ordos basin.This article takes Majiagou formation of lower paleozoic in Fuxian area of Yan'an gasfield as the research object.Based on the fine stratigraphic division,through a number of analytical tests has been done,such as the photographs of outcrops and cores,rare earth elements,light and heavy minerals,mercury intrusion,which are used to study on the development occurrence,sedimentary facies types and distribution characteristics,paleo-geomorphic and paleo-structural uplifts of the main Stratigraphy in the area.This paper analyzes the petrological characteristics,reservoir space feature,diagenesis of the reservoir,and evaluates source rock and reservoir types comprehensively.Finally,a comprehensive study has drawn the control factors for gas reservoir,and prevides theoretical basis for the exploitation of tight carbonatite reservoir of Majiagou Formation.The achievements and conclusions are listed as follows: 1.The paleo-geomorphic unit is quantitatively restored by the "dual interface" method,this paper obtains results that the flanking of erosion ditch and the annular edge of erosion colliculus have better substance characteristic than other geomorphic units,such as shallow depression?the bottom of erosion ditch;2.Most of dolomite in research area are metamorphic dolomite formed in a dry heat reduction environment,but some of them had been influenced by meteoric water leaching;3.According to the reservoir physical conditions,capillary pressure curve characteristics,pore structure type and other parameters,the reservoirs in the study area are divided into four categories;4.There is good fitting relationship between storage factor and productivity of gas testing,which can provide a basis for the later development of wells and stratigraphic selection;5.There are three types of source-reservoir-cap combinations in the area: “sandwich”,transfer,and inverted,and the main type is inverted,the others are supplemented.The research in this paper clarifies the stratigraphic characteristics of the top assemblage,establishes a new restoring paleo-geomorphology method,finds the spatial pattern of tuff and favorable reservoirs,and establishes the correspondence between physical conditions and productivity,which provides a reference for the identification of reservoirs and the estimation of productivity.