Deep subsidence of the early catagenesis zones of organic matter in the sedimentary rocks of Northeastern Sakhalin shelf at high thermal regime of the basin

Abstract

Heat flux measurements in the shelf zone of Northern Sakhalin reveal strong heating of the sedimentary complexes with a temperature gradient of the order of 36 ° C/km along the entire length of the Okhotsk Sea margin of the North Sakhalin basin [2]. However, despite the high thermal regime of the lithosphere in the region, the fact that initial stages of catagenesis gradations PC3‐MC1 ( R o = 0.50‐0.64%) are confined to great depths (3800‐5000 m) is a characteristic property of the deep distribution of the organic matter catagenesis of sedimentary rocks in the boreholes of Northern Sakhalin. It is likely that this fact is related to a high rate of sediment accumulation in the Neogene-Quaternary time. In order to check this supposition and estimate the rate of variations in the degree of transformation of organic matter (OM) in the deeper layers of the basin, we applied the modeling system of GALO basins and reconstructed numerically the history of subsidence and temperature evolution of the rocks of the sedimentary cover in the northeastern part of the Sakhalin shelf in Kaigano-Vasyukanskii area of Sakhalin-5 region (Fig. 1). In addition to the traditionally considered Cenozoic complex, our model of the sedimentary section of the basin includes Upper Cretaceous formations, whose thicknesses were estimated from the seismic data (table). The peculiarity of the shelf region considered here was in the fact that it was located on the one hand within the continental margin of the Okhotsk Sea Plate and on the other hand near the complex of the accretion prism of Eastern Sakhalin [1]. Although the close location of the accretion cline (and consequently the underthrust zone) predetermines the domination of compression stresses over the territory of the basin and consequently the nonisostatic response of the basin lithosphere to the load of water and sediments, there is sufficient evidence of active fracture tectonics in the northeastern shelf of Sakhalin with notable weakening of the lithosphere in the Late Cretaceous and Cenozoic (see below). Therefore, we considered two limiting versions of the basin development. In the first of them, we assumed that numerous fractures and displacements along the fractures significantly weakened the lithosphere strength in the region studied. Under these conditions, the local isostatic response of the lithosphere to the load of water and sediments existed during the entire history of the basin, which we consider started in the Late Cretaceous (and possibly excluded a few relatively short periods). In the second alternative version of the basin formation, we assumed that the local isostatic response of the basin lithosphere to the load of water and sediments existed only during the last 34 Ma starting from the time of the Kuril arc formation, which absorbed the main part of the regional compression stresses. The initial heat flux in the basin in the second version was assumed notably lower than in the first one. Of course, in both versions of the basin development, modern temperature distributions and OM maturity with depth (the values of the reflecting property of vitrinite, R o , %) calculated in the model were close to the values measured in the regions of the basin considered here and in the neighboring regions (Fig. 2). The main modeling results of the thermal history of the basin and variation in the OM maturity are shown in Fig. 3 for the first version of the basin development, which assumed an isostatic response of the lithosphere to the load of water and sediments and consequently high heat flux (approximately 160 mW/m 2 ) in the beginning of the Late Cretaceous. In such a model, intense subsidence of isotherms in the Pre-Cenozioc period of the basin development (Fig. 3) is determined