Sequence Stratigraphy and Sedimentary Environment Identification Based on Palynological Analysis

Abstract

Abstract The subsurface identification of sequence stratigraphy and sedimentary environment commonly has a high uncertainty due to the ambiguity, low-resolution, or discontinuity of seismic data, wire log data, core description data or outcrop data. However, palynology can provide a relatively precise paleoecological and paleoenvironmental results and thus can be used as an explicit lateral correlation indicator for geologists. This paper shows a sequence stratigraphy and sedimentary environment determination case primarily based on palynology for the Upper Main Pay Member in Rumaila oilfield in Iraq. 640 samples in 8 wells were analyzed and grouped which occupied 93 palynological spices. They were clustered into 6 groups, including terrestrial algal group, arid terrestrial taxa group, wetland terrestrial taxa group, least marine dinocyst (dinoflagellate cyst) group, medium marine dinocyst group and most marine dinocyst group. The vertical and lateral changes and its rules of total counts and diversities for each group were studied after appropriate statistics based on their similarities and facies intervals. The distribution and variation of each groups from bottom to top and from proximal-source to distal-source showed some interesting and regular trends which could be associated to the results of sequence stratigraphy evolution and sedimentary environment switch. The arid terrestrial taxa group was a good indicator of the distance from the sea and of the transgressive process, which implied a continuous transgression process in the study area. The wane and wax among the least marine, medium marine and most marine dinocyst groups manifested a relatively variable salinity paleoenvironment. The simultaneous occurrence of peak total counts of the least marine dinocyst group and terrestrial algal group, combining with the relatively low total counts of the arid terrestrial taxa group, suggested a barrier island paleoenvironment. In total, the variety of the palynology in lateral and vertical dimensions recommended a transformation environment from delta to estuary, which is very difficult to be determined by other kinds of data. After the reconstruction of sedimentary environment, the influential factors, the sources (allogenic or authigenic) and the transportation processes about the terrestrialalgae, spores and pollen, and dinocysts were studied, integrated with energy distribution of estuary model. The grouping and interval samples summing statistics of palynological spices showed perfect assistances and verifications for the sequence stratigraphy determination and sedimentary environment identification. This could be helpful for lowering the uncertainty of geological studies, especially for the complex and ambiguous formations.