Akchagylsky-Absheronsky deposits in Northern Caspian region: features of structure, evolution, and oil and gas occurrence

The authors discuss new or insufficiently covered aspects of geological structure of the Akchagylsky-Absheronsky deposits (Northern Caspian) with the use of large seismic and geological data volumes. Basing on them, understanding of their structure, conditions of formation and gas occurrence was considerably improved. The updated paleogeographic charts were created showing that the Akchagylsky and Absheronsky water bodies together with the Southern Caspian were the inland seas/lakes, and their sizes exceeded the present-day Caspian Sea. In the Northern Caspian, pre-Pliocene erosion benches determined boundaries of water bodies. It is found that shelves around the basins are composed of the shallow-water terrigenous deposits having clinoform structure. In the Akchagylsky time, shelves occupied about half of the basin area, while in the central regions at the depth of 300 to 400 m they are represented by the depression argillaceous-carbonaceous gas-source rocks. In the Absheronsky time, almost the entire basin was compensated by terrigenous clinoforms exclusive of the narrow central trench. The seismo-stratigraphic sections are demonstrated with clinoform shelf and depression uncompensated deposits. The previous exploration activities in the Northern Caspian allowed identifying numerous fuel gas shows in the Akchagylsky-Absheronsky deposits, often having the commercial flowrates. However, because of the unclear structural geometry, the substantial gas reserves could not be discovered, and the total estimate of the North Caspian gas resources was moderate - about 50 BCM. The revealed features of the deposits under consideration allow for an order of magnitude increase in resources. Gas pools are predicted in non-structural traps composed of mainly Absheronsky clinoform sands. The depression Akchagylsky and Absheronsky clay with Organic Matter content up to 1.5% are the gas sources as well. The Absheronsky sands are excellent reservoirs. It is shown that modern seismic surveys should be used for gas fields exploration in the non-structural traps of the Northern Caspian. Subsequently, their results will be a basis for identification of “bright spots” that can be with reasonable confidence related to accumulations