In early times Sakhalin was Chinese. It was first visited by Japanese in 1630 and was first settled by Russians in 1857. It came entirely under Russian control in 1875, when Japan gave it up in exchange for the Kurile Islands. It was occupied by Japan in 1905, and its southern part was granted by treaty to Japan in that same year. This southern half was returned to Russia in 1946 (Webster's Geographical Dictionary, 1969).
Southernmost Sakhalin between Aniva Bay on the east and the Gulf of Tatary on the west is a peninsula some 50 km wide. The study area is the Susunay depression in the eastern part of this peninsula. There the Cretaceous is beneath a pile of Neogene deposits 2.5-3.0 km thick. The Cretaceous itself is 1.5 to 2.0 km thick on the west and pinches out on the east. Absence of Paleogene sediments in this section suggests that a high was present here during that time. The Central Sakhalin fault extends north-south through Susunay depression.
Active tectonic movements took place at the Pliocene-Quaternary boundary with the result that at the end of the Pliocene the entire area was uplifted above the sea. A model of the geologic history of the study area was based on paleo-density and paleo-temperature reconstructions along seven east-west seismic profiles across the peninsula at a spacing of 20-25 km.
Maximum sedimentary thickness was attained at the end of Maruyam (early Pliocene) time. This was also the time of maximum heating of the sedimentary section. Determination of the position of the oil and gas windows in the modern section was accomplished by superimposing the Maruyam paleo-temperature section on the modern geologic section.
Maximum paleo-temperatures were plotted on maps of the top and base of the Cretaceous. The upper, biogenic gas window corresponds with temperatures of 50-90 degrees C (MK-1 catagenic stage), and the oil window with temperatures of 90-130 degrees C (MK-3 stage). Gas and condensate are generated at higher temperatures. Commercial oil does not occur at temperatures greater than 200 degrees C.
The predictive maps indicate that oil pools may be present in the upper part of the Cretaceous in the western part of Susunay depression and gas pools in the eastern part. Close to the base of the Cretaceous section in the east of the depression the temperatures correspond with the oil window, and on the west they correspond with the thermal gas window. (Taken from Isayev and Volkova, 1998; digested in Petroleum Geology, vol. 35, no 1, p. 39-44, 2001, 1 cross section and 3 maps, in preparation)
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