U. S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
Sandra J. Lindquist
Open-File Report 99-50-O
Province Boundary and Geographic Setting
Sakhalin Island is part of the northwestern Pacific rim, adjacent to the southeasternmost coast of mainland Russia, directly north of Japan’s Hokkaido Island, and between the Sea of Okhotsk and the Tatar Strait (fig. 1). The North Sakhalin Basin geologic province includes much of the northern half of the island plus northwestern (Baykalo-Pomor syncline) and northeastern (North Sakhalin and Pogranichnyy grabens) offshore areas (red outline on fig. 1). The 84,000-sq-km province area (72% offshore, 28% onshore) is within latitude 47.5° to 55.5° N. and longitude 140° to 146° E. Southwest of the province are the onshore East and West Sakhalin uplifts, the offshore Tatar Strait and Terpeniya Bay Basins, and the Sikhote-Alin Folded Region of the Russian mainland. East of the province is the offshore Deryugin Basin.
North Sakhalin Basin is a deep (to 8 km), Tertiary strike-slip downwarp associated with the major, north-south trending Hokkaido-Sakhalin-Kashevarov en echelon dextral shear system (Mochalov, 1983; Worrall and others, 1996). The basin is filled with Paleocene and post-Paleocene siliciclastic marine sediments and eastward-prograding deltaic deposits of the paleo-Amur River (fig. 1 and fig. 2).
Sakhalin Island and most of the North Sakhalin Basin unconformably overlie Cretaceous to Paleocene deformed and metamorphosed accretionary rocks of a complex continental suture (fig. 2 and fig. 3a), including flysch, blueschists, melange and ophiolites. In westerly and northerly directions, approximate age-equivalent paleo-Amur strata are underlain by partly conformable Cretaceous to Paleocene flysch and forearc strata and by volcanic and intrusive rocks that crop out locally on western Sakhalin Island and on the Russian mainland. East of the suture zone (east of Sakhalin Island and under the Okhotsk Sea), Eocene to Recent strata are underlain by acoustically distinct basement rocks of the Okhotsk crustal block that collided with the Bureinsk massif.
NE-SW trending normal faults (Eocene to Early Miocene transtension) and slightly younger, NW-SE trending en echelon thrusts and folds (Late Miocene and Pliocene transpression) complement the major N-S vertical dextral shear faults of North Sakhalin Basin (figs. 3b, 3c and 3d). Most known hydrocarbon accumulations along the East Sakhalin shear zone of the island’s eastern side are associated with these structural features, especially those of compressional origin .
Early Tertiary transtension provided necessary accommodation space for deltaic progradation from the paleo-Amur River and its distributaries. Depositional rates were as high as 500-800 meters per million years (Nikolayev and Kleshchev, 1984; Tull, 1997). Continued wrench movement likely contributed to the strike (N-S) dispersal of sediments. Late Pliocene tectonism and orogenic inversion resulted in significant geologically recent folding, in modification and rupturing of pre-existing structures, and in uplift of the western and some central regions while other areas were subsiding (Mochalov, 1983; Tull, 1997). Offshore regions were less tectonically deformed than those onshore. Pliocene tectonism resulted in local onshore erosion of as much as 3.5 km and largely created the physiographic configuration of the province today. The Pleistocene Epoch was characterized by extension and transtension, which served to breach traps that contained accumulated hydrocarbons.
The North Sakhalin Basin’s overall structural configuration is compatible with modeled stress fields and complex strain signatures resulting from the collision of India and Eurasia, in which sinistral and dextral wrench systems act as regional conjugate shear sets (Worrall and others, 1996). Major sinistral shear systems are just north of the North Sakhalin Basin Province, and some dextral systems experienced sinistral movement in their past. Tectonism can be related to intermittent magmatic movements in the crust and mantle (Sychev and others, 1986) and to crustal microplate drift in the northwestern Pacific Ocean. Present thermal phenomena, mud volcanoes, and seismic activity are evidence of active movement on many faults.
and Discovery History
Offshore fields were discovered beginning in the 1970s. The Pogranichnyy trough (east of the central part of the island, fig. 1) was first explored by deep drilling from 1971 to 1975, and southernmost Okruzhnoye field within that trough was discovered in 1972. Offshore exploration and development occurred jointly with Japan between 1976 and 1982 and included discovery of the Chaivo and Odoptu fields, 2nd and 4th largest in terms of province reserves. Largest reserve volumes were added by field discoveries from about 1976 to 1986. The six largest fields (five of which are offshore) were discovered since 1975, but the next three largest fields (all onshore) are among the earliest discoveries made prior to 1936. Potential significant Eastern Asian markets for Sakhalin oil and gas include Japan, Korea and China.
All existing offshore fields are in water depths of less than 100 m. Ice conditions in the Sea of Okhotsk have challenged both exploration and development efforts. Typical 2-m-thick ice floes can move at speeds of 1 m/sec, and ice routinely scours the sea bottom.
PETROLEUM AND SOURCE
Source rocks are Middle to Upper Miocene alluvial, deltaic and prodeltaic marine shales, coaly shales and coals, which range from oil- to gas-prone, and Upper Oligocene(?) to Lower Miocene deep marine, diatomaceous, oil-prone shales (fig. 5). The formations in which the source-rock shales occur are hundreds of meters thick. Effective source rocks are somewhat less abundant in the more western, sand-rich facies of each formation, and overall source rock character changes from mostly gas-prone in the western onshore regions to oil-prone in eastern offshore areas (Huizinga and others, 1997).
Some published literature attributes significant hydrocarbon reserves to the deep-marine siliceous-shale source rocks. For example, Kodina and others (1989) discuss the isotopic similarity of Lower and Middle Miocene oils (d13C = -25.2 to -25.8 o/oo) to Lower Miocene bitumen extracts (d13C = -26.4 to –23.9 o/oo), and state that they both exhibit characteristics compatible with deep-marine diatomaceous oozes and siliceous source rocks.
Others believe that petroleum in the northern part of the province is sourced by the older, Upper Oligocene to Lower Miocene proximal-to-distal deltaic shales (rather than anoxic, deep-marine siliceous source rocks), based on biomarker, isotopic and chemometric oil-source correlations (Peters and others, 1997 (abstract only)).
Variable proportions of humic and sapropelic organic matter in the source rocks – related both to age and to paleogeographic setting (marine to continental) – result in differences in petroleum geochemical character, including normal sterane (C27, C28, C29) ratios, cyclohexane to cyclopentane (ch:cp) ratios, and pristane-phytane (pr:ph) ratios, according to Popovich and Kravchenko, 1995, and Tull, 1997. These researchers believe that the northern part of the province is dominantly sourced by Middle to Upper Miocene shales (largely sapropelic), with petroleum characterized by subequal normal sterane content, ch:cp of 0.26-1.28, and pr:ph of 1.1-2. In contrast, they assert that central and southern parts of the province are sourced primarily by uppermost Oligocene and Lower to Middle Miocene (mostly humic) siliceous shales, with the petroleum characterized by C29 dominance, ch:cp > 1.5, and pr:ph of 1.13-2.61.
Oil from the fractured, siliceous reservoirs in Okruzhnoye field (Pogranichnyy trough coastline) is characterized as low density (0.8 grams/cubic centimeter), high resin (20%), low-sulfur (0.26%), and low-paraffin (1.8%) (Tyutrin and others, 1982). The associated gas is typically 70-91% methane.
Gas data from Kalendo and Tungor fields (onshore northeastern Sakhalin Island) show methane ranging from 78-97%, C2+ ranging from 2-7%, CO2 ranging from <1-15%, He ranging from 10-26 ppm, and N2 at <1% (Kamenskiy and others, 1975).
Migration paths include short to moderate lateral distances and significant vertical distances along faults, particularly along the major regional shears. Pleistocene leakage along these faults, especially from onshore accumulations, has resulted in many of those traps being underfilled relative to their spill points. Easternmost offshore basinal areas in the province expelled hydrocarbons westward and contributed to creating local overpressures.
TRAP STYLE AND DEVELOPMENT
Large, low-amplitude structural closures began to form in Early Miocene time, and more intense syn-sedimentary folding had occurred by the end of Middle Miocene time (fig. 5). The overall basin axis shifted progressively eastward throughout the Tertiary Period (Mochalov, 1983, 1985). Late Pliocene high-amplitude folding and inversion and later Pleistocene extension resulted in local loss of trap integrity and the redistribution or leakage of generated hydrocarbons. Thus, many onshore traps are not filled to spill point, but anticlines reportedly are less faulted in eastern offshore regions.
The northeastern part of the province is characterized by local overpressures (20% above normal; Tull, 1997) and by hydrodynamic impact, particularly in eastern areas where coastal or marine sandstones have a lithologic transition into offshore shales. This combination of phenomena causes oil-water contacts dip significantly westward in several fields.
North Sakhalin fault displacements range from tens to thousands of meters vertically and horizontally. Structural closures are characterized by areas of 5-300 km2 and amplitudes of 80-600 m (Mavrinski and Koblov, 1993; Nikolayev, 1983). Some of the best anticlinal traps are reported to be associated with intersections of faults (Saprygin and others, 1978).
The Baykalo-Pomor synclinal area (offshore and onshore) on the northwest side of Sakhalin Island (fig. 1) was in existence by Middle Miocene time, and it contains eight major anticlinal zones and numerous folds with maximum dimensions of 30 km in length, six km in width, and 600 m in amplitude (Mustafin, 1983). Further Neogene structural deformation was contemporaneous with sedimentation. The structural closures are complicated by strike-slip (both dextral and sinistral) faults, with lateral displacements to 30 km, and by normal faults. There are no published penetrations in the offshore portions of the Baykalo-Pomor syncline, and less is known about the existence or extent of folds offshore.
The Pogranichnyy trough (south-central part of the east portion of the island and its adjacent offshore,fig. 1) has little published information, except for the Okruzhnoye field area at the coastline where traps consist of multi-directional block faults and thrusts (Silverman, 1990). Extension of paleo-Amur deltaic facies into the Pogranichnyy trough is questionable.
All Tertiary formations generally are more shale-rich and more marine in origin to the east. Reservoir sandstones and pay zones are commonly stacked. For example, East Ekhaba field (northern coastal area, schematically shown on fig. 3d) contains 18 hanging wall and 20 footwall Miocene sandstone pay zones (Nikolayev, 2000). Producing sandstones range from laterally continuous (shallow-marine deposits) to highly discontinuous (channel deposits), with maximum individual thicknesses as great as tens of meters, but more typically several meters. East Ekhaba channel sandstones trend east-west and are 0.1 to 0.4 km wide. At Mongi field (central coastal area), offshore-bar reservoir sandstones have maximum dimensions of 3.6 km by 14 km (Gololobov and others, 1983).
Many sandstone reservoirs associated with this active Tertiary margin are mineralogically immature. Hanging-wall sandstones from multiple formations in the East Ekhaba field are fine to medium grained, with 30-45% quartz, 15-57% feldspar and 10-27% rock fragments (Nikolayev, 2000). Okobykai and Dagi sandstone reservoir rocks in the Gilyako-Abunan field (northern onshore area) contain frameworks of quartz, feldspar and chert, with cements of chlorite, kaolinite, carbonate, and quartz (Kuklich and others, 1984). Common montmorillonite clays convert to illite and mica with increased depth of burial.
Fractured siliceous shales that form the "silicite" reservoirs have been described for Okruzhnoye field (Yurochko, 1982; Danchenko and Chochiya, 1983), which also produces from younger Miocene sandstones and contains a 600-m oil column. Many silicites formed from globules of oversaturated gels during gas-hydrothermal stages of subaqueous volcanic activity. The Pilengskaya formation at Okruzhnoye field is 100-500 m of siliceous and clay-siliceous rocks. It contains montmorillonite and illite clay, tuffaceous pyroclastic or terrigenous quartz and feldspar, and authigenic silica as globules, with lesser amounts of pyrite, siderite, calcite and glauconite. Much of the silica was derived organically from diatoms and sponge spicules. Pilengskaya silicites include opoka or opoka-like rocks (cristobalite globules with <1 to 4 micron pores, 50% of formation), chalcedonite (chalcedony with pores <1 micron, 5-10% of formation), siliceous argillites (cristobalite and opal with pores < 1 micron, 35-40% of formation), and some diatomites. All silicites contain both tectonic and diagenetic joints.
Underexplored reservoir rocks in the North Sakhalin Basin Province are eastern (offshore) Miocene sandstones of deep-marine origin, fractured siliceous shales, and pre-Tertiary serpentinites.
Reservoir quality figures have been published for several fields producing from Miocene sandstone reservoirs. East Ekhaba field sandstones have 14-36% porosity in the hanging wall, and even "better" reservoir properties in the footwall (Nikolayev, 2000). Mongi field reservoir sandstones are characterized by 20-25% porosity and 3 darcies permeability (Gololobov and others, 1983). Gilyako-Abunan field has porosities up to 40% and permeabilities up to one darcy at less than 1500 meters paleodepth, but at 3500-4000 meters paleodepth, porosity is less than 10% and permeability less than one millidarcy (Kuklich and others, 1984).
The fractured siliceous shale (silicite) reservoir rocks can have "interglobular" porosity ranging from 3-27% , but matrix permeabilities commonly are < 0.01 md (Saprygin and others, 1978; Margulis, 1996).
ASSESSMENT UNITS (AU)
Future fields will be in Middle Miocene to Pliocene sandstone reservoir rocks and in self-sourced Upper Oligocene to Lower Miocene fractured siliceous deposits comparable to the Monterey Formation of California. Some future reserves might be from pre-Cenozoic basement rocks that are unconformably overlain locally by Tertiary source rocks. Hydrocarbons will be found in anticlines, fault traps and stratigraphic traps. The expected total drill depth is approximately 3500 m for future oil fields and 6000 m for future gas fields. Future gas fields are expected to outnumber future oil fields by a 2:1 ratio. Province water depths do not exceed 200 m. No reserve growth factor is used in the assessment.
U. S. Geological Survey Open File Report 99-50O