Central Uplift Area Northwest Of The Tarim Plate - Late Ordovician Reef Phase Paleoecology And Paleoenvironment

Northwestern realm of the Central High was a ramp setting with a northern slope and high-angle slope break from the eastern part during the Ordovician time. Ordovician lithofacies shows as sequences of richer dolomites from the Lower Stage and limestones from the Middle-Upper Stage. Ordovician platform rocks covered by Taklimakan Desert are considered as the key targets for oil-gas exploration. Sequences throughout from the Yijianfang Formation (Darriwilian, Middle Ordovician) to Lianglitag Formation (Kaitian, Upper Ordovician) are multiple occurrence of reef complexes units with lithological differentiation are notable aspect controlled by the palaeoenvironmental pattern. Identification of the biofacies and the division of ecological units from individual well are potential database favorable for the outline of the idealized reservoirs style. Lithological parameters and taphonomic characters such as grain sizes, sotting, rounding, breakage, stacking density indexes as well as their ratio of the grain and matrix are of the environmental proxies of proximal and distal transportation. Reef units have most reef-building communities with in situ frameworks elements. However, classification scheme of banks prefers to their dominated components and fabric of grain; they are catalogued as sandy-sized intraclastic debris type, crinoidal debris type, oolitic and oncolitic debris type, sandy-sized intraclastic and bioclastic type, etc. Synthesized palaeoecological analysis of benthic associations are recognised and attributed to the depth ranges. Paleoecological correlations along the wells show the temporal and spatial distribution of the divise pattern of the ecological units which were obviously controlled by the depth and energy.Along the Lianglitag Mountains, east of the Bachu County, the Darriwilian Yijianfang Formation to Kaitian Lianglitag Formation are about 300 m thick dominantly composed of shallow marine limestones. Altogether six episodes of reefal buildings with variety in biotic structures are recognized throughout the sequence, five of them are microbial-calcareous algal dominated. Only one of episodes yielded from the Yijianfang Formation is typically of Calathium-microbe reef mounds. The Yijianfang Formation could be subdivided into three members lithologically and yields three units of reefal rocks intercalated by sandy bioclastic limestones. First and second reefal units are respectively illustrated as biostromal extended algal mats together with thrombolitic debris. The middle member rich in lithoclastic packstones, calcimicrobe bindstones, bioclastic grainstones and Calathium bafflestones show coeval time occurrence and stable horizontal extension of the reef mounds. The Well Gucheng 4 is located at the eastern margin of the Central High of the Tarim Block with thickness of 130 m from the Yijianfang Formation show as the reef mound community composed of Epiphyton-Renalcis-Calathium from the lower part and bank association of sandy clastic grainstones to bioclastic packstones and wackestones from the upper part. Well Luoxi 1 is located at the inner slope belt of the western Luoxi Carbonate Platform, eastern Tarim Basin. Yijianfang Formation limestone about 330m thick show that microbial mound is yielded from the middle - lower part of the Yijianfang Formation showing as algal-dominated biotically. Intercalated microfacies of laminated cyanobacteria, thrombolites as well as laminated algal-debris illustrate that rich algal layers occur throughout the unit. Stromatotactis structures are commonly occurred. Reefal communities formed by algae or/and metazonal frameworks of this episode have a wide spatial distribution from diverse palaeogeographic settings. The Sanbian (Upper Ordovician) Tumuxiuke Formation marks the acme of the sea-level rising at the beginning of the Late Ordovician.Reef complex from the Lianglitag Formation (Katian, Upper Ordovician) marks gradually regression and resurgence of the ramp from shelf environment. The palaleoecological catalogue is recognized as the lower-divise Bachu Type and high-diverse Tazhong Type base on the biotic structures. Reefal biotic structure in Bachu outcrops are typical of algal-mounds with diverse of morphological profiles. They mostly show sharp margin and clear relief morphologically, and could be catalogued to small-middle scale in size from about one to ten meters thick and several to tens meters in lateral extent. Peloidal, micritic and calcified microbial crusts and encrinital limestones are ubiquitous in association with metazoanal framworks colonize as baffler and microbes would be binding, trapping increasing amounts of sediment and providing favorable conditions for the reef growth. These structures of cryptomicrobial origin with a simple form offer crudely or delicately laminated crusts. Microbial mud mound was formed by microbials binding calcareous algaes, peloids, clasts etc, and called as the Bachu Type. Small-scale carbonate mudmounds are sparsely occurred. Differentiation of the reef mound size shows a depth variety control illustrating as bigger mounds are concentrated in the north region; in contract, small mounds as well as shallower algal-mats and nautioloidal-brachiopodal banks from south region. However, employed well cases of the Tazhong 45, Tazhong 63 and Tazhong 35 illustrate that a higher diverse biota did exist from the Central Tarim High include mainly microbialites, calcareous algae, foraminifera, sponges, corals, stromatoporoids, bryozoans, ostracodes, trilobites, gastropods, brachiopods, crinoids etc. Two reef types, mud mounds and stromatopora - aglae reefs are mainly occurred from the Lianglitag Formation. The stomatopora-aglae reefal communities from the Lianglitag Formation could be typically viewed as the biotic profile of the Late Ordovician reef evolutionary pattern and implies a higher bio-diversity of coral-stromatoporoid-calcareous algal reefal evolutionary taxa. Based on the six well cases between the Bachu outcrop and Central Tarim High, the ecological boundary is outlined. Wells Fang 1 and He 4 is more similar to the Bachu outcrop with microbial style in reefal biota. Well He 3 as well as wells Ma 401 and Ma 5 are typically correlated to the metazoanal-dominated reef structure of the Central Tarim High. Reefs are minly domimated by bryozoan and calcareous algaes binding peloids, bioclasts, intraclast etc. Stomatopora-aglae reefs are mainly composed of microbials binding stromatoporoids or calcic algae bafflestone with stomatoporoids.