Researches on the karst stage of Ordovician carbonates in the Tahe Oilfield have been ceaselessly renewed.Predecessors considered that there were mainly mid-Caledonian(Episode Ⅰ,Episode Ⅱ) karst and early-Hercynian karst in the Tahe Oilfield.However,through the study of petrology,sedimentology and seismology,this paper concluded that the Episode Ⅲ karst of mid-Caledonian existed in the Tahe Oilfield.The evidence indicated a series of recognition markers,such as there developed a corroded valley of a NS-trend paleo-river at the top of the Sangtamu Formation and the residual thickness of the formation within the Silurian coverage became thinner from southeast to northwest,etc.The main layer and area affected by the Episode Ⅲ were in the middle-lower Ordovician pure limestone stratum and the northwestern paleo-geographic high,respectively.Restricted by an aquifuge of the Sangtamu Formation and the global ice age from the end of Ordovician to early Silurian,this karst was represented by an exposed weathering crust and the karstification occurring along fractures,respectively in the north and the south to the wedge out of the Sangtamu Formation.

The Ordovician formations of Tahe Oilfield mainly undergo two karsts of mid-Caledonian and earlyHercynian.The predecessors considered that because of the thick aquifuge in Sangtamu Group(O3),the effectingrange of early-Hercynian karst was controlled by the edge line of the Sangtamu Group: The Northern part which lackNo.3 Abstract Vthe Sangtamu Group was the affecting area of early-Hercynian karst,while,the Southern part which was coveredby the Sangtamu Group was the the affecting area of only mid-Caledonian karst.Recent research shows that for thereason of its favourable factors,such as high-uplift amplitude of structure,long time exposure,highly developmentof fractures,fluent hydrodynamic etc,it is extremely beneficial to karst effections.On top of that,the early Hercyniankarstis able to break through the restriction of the Sangtamu aquifuge.The coverage of the Sangtamu group onlychanges the model of early-Hercynian karst:from crust karst to Covered-Pressurization karst.The discovery of thiskarst has a great guiding significance to the exploration for cavity-reservoirs in the southern and southwestern areas.It points out that,only the fault zones which are connected with the main region can be the favourable reservoirzones,otherwise,Reservoir would be less developed.

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.

The pore evolution of carbonate is controlled by all kinds of diageneses, in which the calcite cementation is one of the most important reasons for pore damage and porosity decline. Diagenetic types and stages in limestone of Lianglitage Formation of Upper Ordovician in TazhongⅠOffset Structural Belt are various and the relationships between them are complex. Based on the analysis and research results of all diagenetic products and structural fabric characteristics by core, thin section, cathodelu-minescence andgeochemistry, three phases and ten kinds of calcite cementations have been identified. The first phase cementation occurred in submarine diagenetic environment and the products are microcrystalline calcite, fibrous calcite, divergent fibrous calcite, fasciculate calcite, radial-axial calcite and subspherical iglite with a pore filling of 0 to 30%. The second phase cementation occurred in atmospheric diagenetic environment and the main products are crescent or pendent calcite, vadose silt, syntaxial cement with a pore filling of 5% to 100%. The third phase cementation occurred in burial diagenetic environment. The crystals of cement are bright and massive and the main types are calcsparite and poikilotopic calcite with a pore filling of 5% to 100%. This paper systematically summarized the influence of cementation on pore evolution and established the history of porosity evolution.

This article has detailedly elaborated the main diagenesis types of Upper Ordovician platform margin reefs and grain banks in TaZhong area and their affection on reef-bank reservoir quality. The carbonatite diagenesis and porosity evolution history have also been established. Calcite cementation is the most important reason for porosity damage and porosity decline. Denudation and tectonic disruption are the extremely important constructive diagenesis for the improvement of porosity-permeability property. Reef-type geomorphic uplift,exposure controlled by sea level changes and developed atmospheric fresh-water corrosion of early diagenesis,leaching and karstification are the essential reasons for high quality reservoir development of platform margin reef banks deposit in TaZhong area. Establishment of reservoir diagenesis and porosity evolution models have disclosed the stages and specialities of porosity evolution,which are of great importance to reservoir recognition and further exploration.

Wellblock TZ16-44 is located in the carbonate platform margin in deposition period of Lianglitage formation of Late Ordovician in Tarim basin, which is characterized by such intrafacies as grain bank, reef mound, lime-mud mound and, interbank sea. The reef bank complex composed of different sedimentary microfacies, becomes major depositional types in this area. The study shows that the reefs inside this area distribute in groups or belts along the platform margin, and moderate-low energy psammitic banks are located in the inner side of the reef body. The reefs, the bioclastic banks and the biological psammitic-psephitic banks that are associated in form of upper and lower combination with them are premium reservoir facies, while the moderate-low energy psammitic banks seperately developed in the backreefs are hard to form favorable reservoir facies. So, the bioclastic banks and the biological psammitic-psephitic banks should be important targets for petroleum exploration in this area.