Controls of Provenance and Depositional Facies on Sandbody Distributions of the Upper Paleozoic in Southeast Ordos Basin

Key words: 
• Ordos basin

Abstract: In recent years, lots of new gas plays are found continuously in the Upper Paleozoic in southeast Ordos basin, which displays great exploration prospects. The Upper Paleozoic was influenced by multisource in the former study, while the main provenance was from north. It is not clear that whether there was sourthern provenance, how much the influenced area was, and what degree the influence was, which greatly influenced gas exploration strategy in the study area. Since lithologic play is the main reservoir in Ordos basin, the study on source and the rule of depositional facies on Upper Paleozoic sandbody distribution is of great importance in southeast Ordos basin. Based on data of well drilling, logging and core as long with outcrop sections of surroundings, by means of paleocurrent direction measuring in outcrop sections, heavy mineral analyzing as well as facies belt diagnosing, the provenance directions of the Permian are confined, the depositional facies styles of the Upper Paleozoic are analyzed, the rule of source, geography of depositional basement and depositional facies on sandbody distributions in the study area are studied, and also the distribution feature of skeleton sandbody are disclosured. Based on the above studies the following conclusion can be obtained. Paleocurrent direction revealed the whole middle part of the study area was catchment area in periods of Shanxi and Shehezi Formation, which suggests the possible existence of north and south of two different sources in the very time. Heavy mineral analyzing indicated that the NNE region of the studied area is rich in zircon, the NNW region of the studied area is mixed area of zircon, leucoxene and garnet, while the south region is rich in leucoxene, which disclosures that the provenance of Shanxi and Shehezi Formation can be divided into three directions which are NNE, NNW, and south. The coast line and geography of deposit basement reflected by thickness of strata and limestones indicated transgression direction being from east. Benxi Formation mainly developed barrier island & lagoon and tidal flat facies, the distribution of barrier island and tidal channel sandbodies were controlled by geography of deposit basement and depositional facies belts with the barrier sandbody being distributed along NNE direction. Depositional facies belts of Shanxi and Shehezi Formation were controlled by the orientation of provenance, which controlled the distribution of under water lobate distributary channels. Shan 2, Shan 1 and He 8 Member mainly developed delta front depositional facies sourced from different directions which were controlled by provenance directions and depositional facies belts, while Yan'an region being the center of water catchment. The skeleton sandbodies are composed of four under water lobate distributary channels converging to Yan'an region coming from NNE, NNE, and SSE as well as south direction, different sourced sandbodies in different layers display the style of overriding in water catchment area. The skeleton sandbodies sourced from south developed well in Shan2 member, which influenced areas of north of Yanan, while the skeleton sandbodies sourced from north developed well in He 8 member, which overreached areas of south of Yanan while the southern provenance receded simultaneously, reflecting that palaeoterrain in the studied area conversed from south being higher than north to north being higher than south, and also indicating that constitution reversed along NS direction in Ordos basin between periods of Shanxi and Shehezi Formation.