Porosity Distribution in the Carbonate Sequence Stratigraphy Framework: A Case Study from Platform Margin in Lianglitag Formation of Upper Ordovician, Tazhong Area, Tarim Basin, NW China

Abstract

Lianglitag Formation has an unconformable contact both with overlying Sangtamu Formation and underlying Yingshan Formation in Tazhong area, Tarim Basin, NW China. Its deposition time is about 3 Ma, so it can be diviede into a third order depositional sequence. The depositional sequence in platform margin can identify the transtressive systems tract(TST) and highstand systems tract(HST), and they can be divide into many parasequences by the boundaries like erosion surfaces, exposure surfaces, hardground and facies change surfaces. Reef and shoal deposite in the platform margin in Tazhong area during Katian, and they experience the eogenetic stage, mesogenetic stage until now. Based on the description of cores, casting thin sections, porosity formed in the eogenetic stage include fabric-selective porosity, such as interparticle, intraparticle, fenestral, growty-framework, moldic, intercrystal, and not fabric-selective porosity, such as channel, vug, carven. The parasequences' thickness is 4.5m-10.5m(mean 8.2m) in the TST. The parasequences have a few eogenetic porosity, less than 10 porosities in each samples. The parasequences' thickness is 1.0m-14.4m(mean 4.5m) in the HST. The main microfacies group in the eastern is grainstone/boundstone contained lots of skeletal grains. The total 1472 eogenetic porosities are identified in 66 samples in T24 and T621well in eastern area, so the average number of each sample's eogenetic porosity is 27. The microfacies group of the parasequences in the middle area is wackstone/packestone/microbial boundstone(few framestone). The total 470 eogenetic porosities are identified in 25 samples in T82 well, so the average number of each sample's eogenetic porosity is 19. The microfacies group of the parasequences in the western area is packestone(or grainstone)/microbial boundstone. The total 105 eogenetic porosities are identified in 13 samples in T82 well, so the average number of each sample's eogenetic porosity is 8. Because the higher content of skeletal grains and faster rate of sedimentation rates, the highstand systems tract develop more eogenetic porosity than the transtressive systems tract. In this case, the eogenetic porosity's thicknesses are 10m∼40m distributed in the upper part of Lianglitag Formation, and develop 1-5 section in different area.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016