Feature Of Vertical Flowing-Conduits System And Their Control On Natural Gas Accumulation In Yinggehai Basin

Thermal fluid activity controlled by vertical flowing-conduits system is one of the important characters of Yinggehai Basin. A series of macroscopic and microscopic geological phenomenon is left in seismic section and rock slice. Many researchers mainly focus on diaper while studying vertical flowing-conduits system in Yinggehai Basin, including diaper structure, characters, oil and gas geochemical characteristics as well as accumulation chronology. However, vertical flowing-conduits system in Yinggehai Basin includes not only blur zoon and associated fracture in diapir structure, but also the widely developed gas chimney structure. The previous various diaper study from the macroscopic aspect cannot satisfied the needs of oil and gas exploration nowadays. It still lacks of study in the spatial difference of vertical flowing-conduits system and its influence on oil and gas accumulation efficiency while scale of vertical flowing-conduits lacks quantitative reference.Through the analysis of thermal fluid vertical flowing-conduits system components and the spatial distribution pattern in center basin, the paper illuminates the type, spatial distribution pattern, internal relation among different vertical flowing-conduits systems and fluid gathering efficiency of vertical flowing-conduits system in the basin by the means of comprehensive interpretation of seismic section,3D property parameters extractions and rock slice observation. Then the paper studies the overpressure calculation problem from four aspects. Firstly, depict current basin stress field structure from the view of point by the analysis of drilling stress structure. Secondly, describe current stress field characters in section and plane with the help of seismic velocity spectrum inversion. Thirdly, recover overpressure compartment structure that was before being transformed through numerical simulation of pressure. Fourthly, analyze vertical flowing-conduits system’s control on current basin stress field by comparison of stress structure change. In other words, the paper describes stress field structure evolution controlled by vertical flowing-conduits system from the view of point, line, plane, body and time. Vertical flowing-conduits system character and stress structure of gas reservoir in different areas help to analyze the distribution regularity of positive gas accumulation zone, which provides theoretical reference for gas exploration in Yinggehai basin. The mainly achievements are as follows.(1) Vertical flowing-conduits system elements and types As formation mechanism is different for flowing-conduits system elements, two vertical flowing-conduits system elements, including diapir secondary fracture and hydraulic fracture, developed in depression of the center basin. Diapir secondary fracture includes trans-tensional fault, arched-extension fault and collapse fault. Hydraulic fracture varies from microcosm to macrography. It is long and thin micro-fracture in rock slice and drilling core while it is cluster output on seismic section extending from500m to1000m and thus forms gas chimney. Fuzzy zone comes into being on the seismic section when many gas chimneys are interlaced in space. According to different space combinations of flowing-conduits elements, vertical flowing-conduits system has been classified into three main categories including type Ⅰ of buried type, type Ⅱ of impale type and type III of collapsed type. Ⅰ type of buried type has three detailed categories including type Ⅰ-Ⅰ of deep buried type, type Ⅰ-Ⅱ of middle buried type and type Ⅰ-Ⅲ of shallow buried type. Top surface buried depth of all these three types’fuzzy zone gradually becomes shallow. On the top, gas chimney and diapir associated fracture developed and for vertical differentiation, flowing-conduits elements of middle-deep buried type are most obvious, type Ⅱ of impale type has two detailed categories including type Ⅱ-Ⅰ of trans-tensional impale type and type Ⅱ-Ⅱ of hydraulic fracture impale type. Top surface of these two types’fuzzy zone is close to seafloor and deformation of the former stratum is obvious while the latter is not so obvious. Type Ⅲ of collapsed type consists of fuzzy zone and collapse fault.(2) Vertical flowing-conduits system regional distribution characteristics and governing factorsVertical flowing-conduits system of center depression is regional in distribution. It is mainly middle-deep buried type in Dongfang area. The early formation of collapsed type and later formation of shallow buried type developed in Changnan area. Shallow buried type, deep buried type and impale type developed in Ledong area. Vertical flowing-conduits system is regionally controlled by three geological factors. Firstly, it is the fierce overpressure environment coming from thick and fine grained sediments in the basin. Secondly, it is the difference in tensile strength and tensile direction between north and south in the basin resulting from the change of regional stress field, leading to weak tension or extrusion environment in Dongfang area. Changnan area is fierce tension environment in early period while later is weak tension environment. Ledong area is fierce tension environment in later period. Tirdly, it is regional differential settlement. There is large thickness difference in the south above T30stratum of the basin, so Ledong area mainly developed impale flowing-conduits system. The first factor is the internal reason that flowing-conduits system mainly developed in central depression while the latter two factors is the external reason of regional difference for flowing-conduits system.(3) The fluid gathering ability and spatial evolution of vertical flowing-conduits systemThe type of vertical flowing-conduits system converted from deep buried to semideep-deep buried, then shallow buried, and then piercement type, and the last collapsing type. Showing from the around exposed stratigraphic, the gas accumulation in the system enlarged first and then reduced in scale, among them the semideep-deep buried type had the largest scale of gas accumulation. The conducting system energy increased first and then decreased as well and the piercement type had the strongest energy nowadays. The conducting system of different types at present have the internal connecting, which presents the different evolvement stages:the deep buried conducting system indicated the initial evolving stage, and then the system stepped into mature stage in two ways of forming the conducting system of deep buried-semideep-deep buried-shallow buried-uprising piercement type under the coaction of level gravity and internal energy or the hydraulic fracture piercement type under the action of internal energy. Finally, the vertical flowing-conducting system may all die out and form collapsing type. Now the vertical flowing-conducting system of Yinggehai is still in constant evolvement.(4) The relationship between the present overpressure structure and the vertical flowing-conducting system distributionThe pressure, as well as the geothermal gradient, of Dongfang and Ledong area in the central depression belt that developed vertical flowing-conducting system was obviously higher than that of northern Lingao area and eastern Lingtou area at the same depth. The overpressure top surface of Dongfang area was relatively deep (about2250m) and the Ledong’is relatively shallow (about1900m). Dongfang and Ledong area all had an increasing pressure structure and the pressure gradient near Dongfang pressure top surface was higher than that near Ledong pressure top surface. The3D overpressure prediction in Dongfang area showed that the buried depth of overpressure top surface increased at the top of the vertical conducting system and slowly decreased in surrounding area.2D overpressure present prediction showed that the overpressure well-developed area had a good corresponding relation with vertical conducting system concentration areas. The present pressure structure of different area in central depression had difference and this difference was related to the vertical conducting system energy of different types, which can be seen that the overpressure scale and intensity in Ledong area which generally developed conducting system of piercement were all higher than that in Dongfang area.(5) The effect of conducting overpressure made on present pressure field.The comparison of ancient stress simulation and the present prediction indicated that the depth of self-source overpressure surface in Dongfang area was4000m and that in Ledong area was3700m, the self-source overpressure surface and conducting pressure surface were all the shallowest near the LD81structure in the central depression and decreased gradually on both sizes. The development of vertical conducting system led to the self-source overpressure surface of Dongfang area increasing for1900m while that of Ledong area increasing for2185m, and92%-92%of the overpressure in the reservoir of Huangliu formation in the depth of2580-3088m in Dongfang while86%-90%of the overpressure in the depth of2580-3088m in Ledong area came from self-conducting overpressure. Vertical conducting system controlled the pressure structure and fluid charging way of the mid-depth and deep reservoir, and resulted that the pressure in sandstone reservoir was higher than the surrounding mudstone.(6) The characteristics of self-sourced and transferring overpressured beltsThe late transform of vertical conducting system made the self-source overpressure surface in central depression increased inordinately, forming a self-source and conducting belt between the initial self-source overpressure surface and present conducting overpressure surface, which was thick in the central basin and thin in the marginal basin and did not exist difference in different area. The comparison results of the Dongfang area, Ledong area and Changnan area indicated that the structure of the self-source and conducting overpressure belt and the size of self-source overpressure were related to vertical conducting system types:the size of the self-source overpressure in Dongfang area was relatively small and the vertical conducting system energy was weak with a well preserved up-caprock, so the self-source and conducting pressure belt had a deep buried depth, short vertical extension distance and quick pressure transition. The self-source overpressure in Ledong area was in middle size, but because of the quick sediment and general developed hydraulic fracture transportation systems up the late T30surface, the self-source conducting overpressure belt had a longest vertical extension and its top buried depth was the shallowst. Nanchang area had the strongest self-source overpressure but the a shallower self-source conducting overpressure belt top surface compared with that in Ledong area for the collapse conducting system formed by late stretching stress.(7) Two favourable gas accumulation zonesThere exists two advantageous nature gas accumulation zones in the central depression of the basin. The first one, at a normal pressure environment, was the top pressure compartment of normal conductive pressure zone, which located up the conductive overpressure surface and formed reservoirs whoes depths were less than2000m. Fracture formed with diapir, gas chimney and fuzzy zone were passageways that connected source rocks and reservoirs in the zones. The top mid-deep passageways in the top pressure compartment of normal conductive pressure zone were more beneficial for forming reservoirs. The second, at an overpressure environment, was in-compartment self-source conductive overpressure zone under the conductive overpressure surface with the buried depth of about2000-5000m. Gas chimney and fuzzy zone were passageways that connected source rocks and reservoirs in these zones. The deep buried and mid-deep buried passage ways were of benefit to forming reservoirs in in-compartment self-source conductive overpressure zone. And the reservoirs in in-compartment self-source conductive overpressure zone were controlled by hydraulic Fracturing passages and had the advantages of not being far from the source rock and having effective fluid conducting. It was the most potential petroleum accumulation zones in the basin.(8) The exploration potential of mid-deep buried nature gas reservoirs in Ledong areaThe mid-deep buried nature gas reservoirs in Ledong area had a good exploration potential, the reasons were as followings:first, Ledong area developed two beneficial nature gas accumulation belts that were similar to those in Dongfang area, and we had made breakthrough in the nature gas exploration in the top pressure compartment of normal conductive pressure zone; second, deep buried conducting system, where gas chimney that was always the best place for gas accumulation formed, developed between shallow buried conducting system and hydraulic fracturing penetration system; the last, the dissolution of thermal fluid in deep formation could play a good role in reservoir physical property transformation.