Diagenesis Framework Related to Distinct Carbonate Platform Types in Tarim Basin, NW China

The Lianglitag Formation carbonates of the Upper Ordovician are important oil-gas reservoirs deep-buried in Tarim Basin, Northwestern China, and are characterized by distinct reservoir types between Tahe and Tazhong, located in the north and centre of Tarim basin respectively. Based on sedimentologic, biostratigraphic and sequence stratigraphic analysis, with the recognition of key stratigraphic surfaces, we define a third-order sequence developed in both Tahe and Tazhong. Five fourth-order sequences were divided in Tazhong, while the lower four fourth-order sequences were only preserved in Tahe. The Lianglitag Formation carbonate platform in Tahe was characterized by ramp and/or weak rimmed types, with low depositional rates (37μm/a). However, the strongly rimmed carbonate platform was developed in Tazhong, with high depositional rates reached 250μm/a and widely development of high-energy reef and shoal facies.

15 and 6 fifth-order high-frequency sequences were further recognized in the Lianglitag Formation carbonate in Tazhong and Tahe, respectively. Correspondingly, distinct (early) diagenesis related to high-frequency sequences are revealed in the studied regions. Many evidences show that early meteoric dissolution and cement are widely developed in Tazhong, such as evidently selective dissolution, cementation in meniscus and pendent fabrics, interparticle cements with low Fe and Mn value, and similar carbon and oxygen isotopic composition between sparry and micrite components. The study shows that freshwater lens developed at the top of a shallowing upward high-frequency cycle, that is, the formation and distribution of early diagenesis were controlled by high-frequency sequence frameworks. The development of early dissolved pores provides primary space and conducting conditions for fluid flow during the further late burial diagenesis, which is an important constraint on the deep-buried carbonate reservoirs preserved in the Tazhong. However, with very weak early meteoric dissolution and relative strong marine cement, the Lianglitag Formation carbonates in the Tahe shows bad reservoirs for oil-gas unless they were modified by late strong fracturing and thermal fluid flow. It is indicated that the above distinct (early) diagenesis frameworks principally resulted from evidently different basement structural activity and terrigenous input on the studied carbonate platforms.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California