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Geology and Oil and Gas Potential in the Kirinsky Block Offshore from Sakhalin Island

Abstract

The offshore area of North-Eastern Sakhalin is explored quite well by the regional, exploration and detailed geophysical surveys. Systematic geological and geophysical surveys were initiated in this region in 1957–1958. They included mostly 2D seismic surveys, by the results of which the following large oil and gas condensate fields were discovered - Chaivo, Lunskoye, Piltun-Astokhskoye, and Odoptu. By the present time, in the Kirinsky block eight deep exploratory wells have been drilled and three gas condensate fields have been discovered in the formations of the Daginsky horizon. Hydrocarbon reserves of the Kirinskoye gas condensate field (GCF) are approximately 180 million tons of fuel oil equivalent. The resource potential of categories C1+C2 is 750 billion cubic meters (bcm). In the section of the region under study, the researchers identify a heterogeneous basement consisting of the Mesozoic formations and a sedimentary mantle consisting of the Cenozoic, mostly terrigenous formations. The sedimentary mantle (having a thickness of 5–6 kilometers and more) is composed mostly of terrigenous rocks of the Paleogene and Neogene. Within the region under study, the Paleogene sequence is identified in reduced volume and is represented by the Oligocene formation, which, according to the regional stratigraphic chart, is subdivided into the Machigarsky and Dayekhuriinsky horizons. The first is composed of inhomogeneous interbedding of gravelstone, sandstone and argillite, which were formed in the shallow water environment. The second consists of argillaceous-siliceous rocks of the outer continental shelf environment. The total thickness of the Paleogene formations here does not exceed 800 m. Hydrocarbon accumulations offshore from North-Eastern Sakhalin were found in the range of the Oligocene to the Pliocene, but mostly confined to the Daginsky horizon formations. Reservoirs are composed by sandy-aleuritic layers formed mainly in the environment of delta, delta front and upper sublittoral. They have porosity 17–25 percent, permeability up to 1 D and more. Within the block, four pay beds were discovered in the Daginskiye formations in the Kirinskoye GCF, 2 beds - in the Yuzhno-Kirinskoye GCF, and 1 bed - in the Mynginskoye GCF. To the West of the Kirinsky block, 19 sand and sandy- aleuritic beds up to 100 m thick were identified in the Lunskoye field, in the mid-lower Miocene interval 1180 m thick. 11 hydrocarbon pools were discovered there.