Ordovician Structural-Depositional Model and Prediction for Profitable Crack Reservoir of Carbonate Rock in Tazhong Area,Tarim Basin
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Ordovician carbonate rock formation is an important target for oil and gas exploration. In this paper, on the basis of regional structure backgrounds , Ordovician lithologic intervals are divided in Tazhong area and, structural-depositional model is built up. With the help of three dimension finite element simulation method, the favorable areas of carbonate fracture reservoirs are predicted. The purpose of this paper is to give a direction for carbonate oil and gas exploration in Tazhong area. Conclusions could be summed up as follows:(1) The lithologic characters of middle- upper Ordovician in the major part of Ordovician uplift in Tazhong are dramatically different from that of adjacent slope, especially from that of slope of south Manjiaer depression in the north of Tazhong number one fault. The lithology of Ordovician in the major part of Tazhong uplift is dominated by carbonate rocks, and the lithology of middle-upper Ordovician is mainly sandy mudstone in the slope of south Manjiaer in the north of Tazhong number one fault.(2) Sedimentation during Ordovician in Tazhong area is controlled by intracratonic depression and Cratonic margin aulacogen, which are with different properties of prototype basin. The uplift and sag in intracratonic depression have prominent control to the sedimentation of Ordovician. Middle-upper Ordovician deposition in the major part of Tazhong uplift is controlled by the large scale anticline in the intracratonic depression formedn early Caledonian (the end of early Ordovician). The restrained platform facies are developed in shallow water in such places where platform clinoform, platform margin, semi-restrained platform (include platform flat, lagoon) and Carbonate morphologic units are found. The northeast boundary of Tazhong area is sharp margin controlled by the early movement of normal fault of Tazhong number one fault. The sedimentation of south Manjiaer depression in north Tazhong is controlled by Kuluketage-Manjiaer aulacogen, the sediments are clastic rocks with slope and deep water basin facies. Middle-upper Ordovician deposition of Tanguzibasi area in south Tazhong is controlled by the syncline of intracratonic depression, the sediments consist of clastic and carbonate rocks with deeper open platform facies. Lower Ordovician in Tazhong area is platform facies controlled by the intracratonic depression, and the structure-sedimentation is no apparent difference between the major part of uplift and adjacent areas.(3) The first favorable fracture zones of Ordovician carbonate reservoirs being predicted in the major part of Tazhong uplift include the following areas:Tz161-Tz44 area, Tz24-Tz26-Tz27 area, Tz45 area, Tz3 area, Tz52 area, Tz403 area, Tz2 area, Tz22 area, the east part of Tz35 area, the west part of number one fault and so on. Correlation the results predicted with the observations of core fractures and well logging evaluating, the fitness is about over sixty-six percent. So it is proposed that three dimension finite element simulation method is very efficient in predicting carbonate fracture zone.Keywords:
Lithology
Tarim basin
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Based on the analysis of the tectonic deformation in the southern Tazhong Oilfield,the present study proposes for the first time a large kink structure as an essential controlling factor to explain sedimentary history and petroleum trap formation in Ordovician carbonates of the Tazhong area.This model is different from previous models that focused on faults and related structures as the key tectonic control on sedimentary sequences and trap formation.The result illustrates that at least three episodes of tectonic deformation occurred in the Tazhong area during the Paleozoic time.The Ordovician sedimentary pattern was essentially controlled by the first episode of deformation that established a symmetrical conjugate chest-like anticline.The topography of the epeiric continental shelf was characterized by an isolated platform surrounded by two symmetrical slope-breaks to the south and north,although the two slope-breaks had different inclinations sloping angles.The platform margin carbonate reservoirs are juxtaposed with the slope source rocks.Such a tectonic setting maintained a balance of subsidence rate and sedimentary compensation rate,thus maintained a shallow marine carbonate platform during the Ordovician.The thick reef complex of the Late Ordovician Lianglitag Formation formed in platform margin.Karst-type reservoirs of the Lower-Middle Ordovician Yingshan Formation in the northern slope and the Kaitian(Upper Ordovician)reefcomplex Lianglitag Formation in the Slope-Break are both considered oil-gas exploration targets in the Tazhong Low-Uplift.
Anticline
Passive margin
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This paper presents the fine interpretation of seismic horizon,analysis of regional tectonic setting,selection of reference datum,applied analysis of moldic method and residual thickness method,recovery of the paleo-relief and fine sculpture of the paleodrainage pattern of the Ordovician unconformity in Tazhong area.These could be as guidesfor well location arrangement.
Tarim basin
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The seismic reflection boundary T~2_7,i.e.,the boundary of the Upper Ordovician Sangtamu Formation and its underlying Lianglitag Formation is interpreted as an important structural- sequence transform boundary in the Upper Ordovician strata on the Tazhong and Tabei uplifts,Xinjiang,and represent the trasition from the Early Palaeozoic carbonate platform-type deposits to terrigenous clastic deposits.The deposits in the two formations cited above were previously assigned to the continuous deposits,and the boundary between them considered as a transgressive onlap and drowned platform.The studies in recent years show that the T~2_7 boundary is a regional fossil exposue surface and depositional break.The truncation is well defined at the top of the Upper Ordovician Lianglitag Formation below the boundary,and the residual thickness ranges nearly from 20 m to 600 m,respectively in the Tabei and Tazhong Basins.The fossil exposure and karstification of the weathering crust permit the development of the karsts in the carbonate rocks from the Lianglitag Formation.These karsts occur as major hydrocarbon reservoir horizons along the F1 fault on the northern margin of the Tazhong uplift .A succession of about 300 m- thick palaeokarst cave systems may be traced across the Lianglitag Formation and its underlying Qarbag Formation,Yijianfang Formation and Yingshan Formation on the southern bank of the Tarim River.These palaeokarst cave systems constitute the magnificent hydrocarbon reservoir rocks composed mostly of the limestone palaeokarst systems from the Upper Ordovician Lianglitag Formation on the southern slope of the Tahe Oil Field.
Onlap
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The shape of carbonate buried-hill palaegeomorphology had an obvious control on the formation,development and effectiveness and oil-bearing property of karst reservoirs.Based on the method of isochronous stratigraphic thickness contrast,a corresponding control-point of ancient altitude was obtained and it was constrained based on fine structural interpretation of unconformity seism,the Ordovician palaegeomorphology in the west of Tazhong Area was restored.In combination with drilling data,the control factors of the development,connectivity and its filling characteristics are studied deeply,it points out the direction for next exploration of oil and gas.
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New Evidence on the Sedimentary Framework of the Early Ordovician in Central Tarim and Adjacent Area
The previous viewpoint considered that,the early Tarim basin was an unified carbonate platform,its slope was developed along the Gucheng-Lunnan area.Based on sedimentologic and seismic stratigraphic data from the lower-middle Ordovician,deep-water slope and basinal facies were recognized along the MJ1-TZ29 section,so that the Tazhong and its adjacent areas could have already been isolated from the unified carbonate platformand seprated into three platforms of Tazhong-Bachu,Gucheng and Tangnan,which was also faced with TaBei area across a trough.The high-energy reef-shore bodies bandingly developed among the edge of each isolated platform as a band,which were mainly composed of mid-high energy grain banks.Inwards,it becomes the deposition of platform,developing the subphases of intraplatform banks,intraplatform depression and interabeach seas and soon.Outwards it becomes deeper-water deposition area of slope-trough.Under this circumstance,the new regional depositional pattern of the Tazhong and adjacent areas has been established.This new pattern has great theoretical and realistic significance for the establishment of new favourable exploration zones.It also greatly increases the strategic position for the petroleum exploration in lower Ordovician of research area.
Tarim basin
Trough (economics)
Deposition
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There exist thick marine carbonate strata of varied lithologic characters in the Lower Ordovician-Upper Ordovician Lianglitage Formation of western Tazhong area. Based on data of drill cores, logs and laboratory tests, the authors recognized three types of karstifications in the study area, namely syn-depositional, weathering-crust, and buried ones. The syn-depositional karstification occurs mainly on the platform margin of Later Ordovician Lianglitage Formation, in which atmospheric lithogenic lens are frequently seen but their sizes are not so big. The weathering-crust karstification is developed in central Kartarke uplift. The buried karstification can be divided into three stages and are distributed in areas where faulted structural belts and igneous rocks are well developed. Lithofacies and sedimentary facies, karstification and structural fissures are main factors controlling the development of Ordovician carbonate reservoirs, which constrain the development of karst reservoir rocks in different layers and positions.
Lithology
Tarim basin
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Tarim basin
Turbidity current
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Based on the new 3D seismic data, the complex structural characteristics and the difference of oil and gas accumulation in Tazhong No.1 Structural Zone were studied with the isochronous sequence framework and structure analysis. It is indicated that Tazhong No.1 Structural Zone is a complicated faulted slope break resulted from a large thrust belt eroded for a long time from the end of the early Ordovician to the early of the late Ordovician. A composite of carbonate reefs and banks of platform margin in Lianglitage Formation of the Upper Ordovician was developed along the carbonate slope break. Tazhong No.1 Faulted Slope Break can be divided into five parts with different geometry and development characteristics by the accommodation of transfer zones,which caused the difference of oil and gas accumulation. The thick composite of carbonate reefs and banks in rimed shelf is located in the Tazhong-44 Block. This area is the favorable exploration district and has high and stable production of carbonate along the slope break. There are intensive structural effect and Karst process in Tazhong-45 Block. This area has an advantageous prospect for a large scale fracture-hole reservoir. The two zones are main exploration targets for large oil and gas fields.
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