The Depositional Features and Distribution Regularities of Marginal platform Grain Shoals of Lianglitag Formation in Tazhong Area

Key words: 
• grain shoal /
• distribution regularity /
• Tazhong No.I slope-break zone /
• platform margin /
• Lianglitag Formation

Abstract: The reef-bank complex of Lianglitag Formation in Tazhong area hosts abundant oil and gas resources. By integrating scientists'  achievement and our comprehensive research on drilling, logging, core and thin section analysis, the categories and characteristics of grain-beach subfacies in Tazhong No.I slope-break zone of Upper Ordovician Lianglitag Formation have been studied systematically. Tazhong No.I slope-break zone is a large rimed carbonate platform margin reef-bank complex sedimentary system, which is composed of four subfacies, including reef mound, carbonate mud mound, grain shoal and interbank sea.  Characterized by diversity, multicycle and large thickness, the marginal-platform grain shoals of Lianglitag Formation in Tazhong area are divided into four types, including intraclast bank,bioclastic bank, oncolite bank and oolitic bank, among which psammitic bank and biological psammitic-psephitic bank are two main types of intraclast bank. It is considered that there are some differences on the category and scale of grain bank in the inner zone and outer zone of carbonate platform margin. The outer zone is mainly composed of high-to-moderate energy grain banks, while the inner zone primarily consists of low-to-moderate energy grain banks. The vertical and horizontal distribution of grain shoals are not isolated, and usually associated with reefs, forming the stacked  modes  vertically, interdigitated horizontally reef-flat complex. One of the  Special  canonical features of oil-gas distribution of Upper Ordovician Lianglitag Formation in Tazhong area is that isolated grain shoal or reef usually cannot form high-quality reservoir, while the reservoir property of grain bank or reef associated with each other is better, and usually hold large amount of hydrocarbons. In the perpendicular direction, it is considered that the superposed characteristics of the carbonate platform margin reef-bank complex of Upper Ordovician Lianglitag Formation in Tazhong No.I slope-break zone display in the form of multiple cyclic assemblages of grain shoals, reefs and lime mud mounds.Individual reef-bank complex or mud-mound-bank complex develops with grain bank at the bottom, carbonate reef mound or(and) lime mud mound overriding it in  its upper part, which is overlapped by the next cyclic grain bank. In the vertical direction, reef mound subface consists of four microfacies, including reef base, reef core, reef flat-reef top and reef cover, on the lateral, reef mound subfacies is composed of four microfacies, including reef core, reef flank, reef front and reef behind. The assemblage of grain shoal and reef subface presents multicyclic characteristics, and the same grain bank can exist simultaneously as the microfacies component part of two different reef mounds located in the vertical orientation of each other, not only can be the reef base of the upper reef knoll, but also can be the reef cover of the lower reef mound. In addition, grain bank can also be one of the parts of reef mound subfacies in the form of reef flat-reef top, reef flank, reef front and reef behind microfacies. Because of the interreef depositional setting of margin-platform reef is composed of high-to-moderate energy grain bank, low-to-moderate energy grain bank and interbank sea sedimentary subfacies, grain bank can occur in interreef subfacies as well. The combination regularity of grain bank and carbonate mud mound has a similar feature to that of grain bank and reef mound. The results show that biological psammitic-psephitic bank and reef mound often exist simultaneously in the form of assemblage in the upper part of the 1st Member and the 2nd Member of the Lianglitag Formation; The oncolite bank and oolitic bank present a property of thin thickness compared with other grain banks, which range from a few meters across to several decametres,and often appear in combination with lime mud mound; The bioclastic bank  (mainly crinoidal or echinoderm bank) usually occurs at reef flank or other terrains which are relatively lower, with high reservoir quality and its thickness ranging from a few meters to over twenty metres or so. The distribution characteristics of grain bank in the vertical and horizontal direction is chiefly controlled by two factors, one is the alternation of hydrodynamic condition caused by the fluctuation of relative sea level, the other is the diversity of secondary palaeogeomorphology of the carbonate platform margin. The marginal-platform grain-beach subfacies presents zonal distribution feature along Tazhong No.I slope-break zone, with its width narrows down from west to east. In the depositional setting of psammitic banks, the other grain shoals spread among them in stripped, blocky or punctuate modalities. In the matter of the categories and distribution regularities of grain beach in Tazhong No.I slope-break zone of Upper Ordovician Lianglitag Formation, different well blocks demonstrate dissimilar particularities. In the depositional setting of psammitic banks, wellblock Tazhong 45 and wellblock Zhonggu 2 are mainly composed of bioclastic banks, with biological psammitic-psephitic banks and oncolite banks distribute among them infrequently, and oolitic banks occur at Well Tazhong 86; Biological psammitic-psephitic banks develop well in wellblock Tazhong 82, oncolite banks exist in the neighbourhood of Well Tazhong 826 and Well Tazhong 82, and oolitic banks appear at Well Tazhong 822; Biological psammitic-psephitic bank and bioclastic bank are two cardinal types of grain shoals in wellblock Tazhong 62, and oncolite banks can be discovered at Well Tazhong 84; In wellblock Tazhong 24-26, biological psammitic-psephitic bank and bioclastic bank are two fundamental categories with oncolite banks scattered among them.