Biota and palaeoecology of the Upper Ordovician Lianglitag Formation in Lunnan Oil Field, Xinjiang, NW China
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Abstract:
Ordovician carbonate deposits in the Tarim Block are considered as key targets for oil-gas explorations. Limestones of the Upper Ordovician Lianglitag Formation in Lunnan Oil Field, Tabei Uplift of northern Tarim Block, are considered to be economically significant reservoirs. The present paper demonstrates biotic and taphonomic features of the limestones in drill cores of 19 wells from Lunnan Oil Field. Index profiles of grain size, sorting and rounding, as well as grain/matrix ratio, are comprehensively described for ecological unit divisions in individual wells; palaeoecological correlations between the wells are outlined. The limestone-dominated sequences of the Lianglitag Formation were deposited on a westward deepening ramp in an epicontinental sea setting, in which high-clarity water and warm and shallow marine-floor were favorable for the thriving of the benthic biota. Intraclasts with diverse components and fabrics were identified under a polarizing microscope; abundant non-skeletal grains and fossils were also examined. Biotic structures are composed of calcimicrobes of Renalcis, Girvanella, Wetheredella，Garwoodia，Phacelophyton yushanensis，Hedstroemia and Halysis; calcareous algae of Vermiporella, Dasyporella, Dimorphosiphon, Plexavaria, Solenopora; metazoans of sponges, corals, bryozoa, trilobites, brachiopods, echinodermata, gastropods, ostracoda and others. Bioclasts are the key components in these limestones. With respect to lithological categories, the limestones are grouped into three microfacies types:Type 1 represents reefal units distinguished by coarse autochthonous bindstones and bafflestones in many wells. Bindstones, dominated by microbial and calcareous algal (e.g., Verminporella, Hedstroemia) fabrics binding peloids, bioclasts and intraclasts, are more common. Stomatopora-algal frameworks occur sparsely in several drill cores. The reefal units are mostly several meters in thickness and low relief morphology. Calcimicrobial forms and calcareous algae are notable components in most reef-building units. Calcimicrobes and calcareous algal taxa are concentrated in the reefal units of the Lunnan Oil Field, indicating that the reefs formed a special ecosystem comparing with global equivalent analogues, which are mostly dominated by diverse metazoans.Type 2 comprises either peloidal or bioclastic grain to packstones, indicating a medium to high turbulent environment. Reworked bioclastic and peloidal grains are abundant, medium to well sorted, therefore, indicating some transportation. Type 3 consists of micritic limestones in several drill cores, indicating a relatively narrow, calm and deeper-water belt. Sparse and fine-sized bioclasts are less than 10% of the rock components. Correlations of the lithological units between well blocks allow reconstruction of temporal and spatial distribution patterns of the Lunnan carbonate platform. Depth changes with deepening towards the west (thus open sea) are considered to have had a key control in configuration of the ecological associations. A western-sloping ramp was thus optimized for reef occurrence. Intra-platform environments in the eastern wells are dominated by diverse intra-clastic banks.
