FEATURES OF THE GEOLOGICAL STRUCTURE OF THE BORISOV ANTICLINAL UPLIFT IN RELATION TO ITS POSSIBLE OIL AND GAS POTENTIAL

B. E. KOPAEV

(Presented by the Department of Combustible Minerals)

The Borisov anticlinal uplift lies within the Suifun synclinal zone, which is bounded by rigid massifs: the Khanka massif to the north, the East Manchurian massif to the west, and the Khasan-Barabash massif to the south. From the east it is adjoined by the Daubikhe synclinal zone.

The oldest rocks participating in the structure of the Suifun synclinal zone are rocks of a folded complex, presumably of Cambrian age. They are gathered into folds of sublatitudinal strike and are broken by numerous disjunctions. Above them lie Upper Permian tuffs and effusives, and still higher—Mesozoic and Cenozoic deposits.

The Mesozoic deposits are represented by three systems: Triassic, Jurassic, and Cretaceous. They are crumpled into large brachyfolds; the bedding of rocks in the brachyanticlines is calmer than in the brachysynclines. Within the complex of Mesozoic rocks there is an unconformity, expressed in the transgressive overlap of Cretaceous deposits onto the eroded surface of older rocks, down to the Upper Paleozoic.

The Cretaceous deposits fill a series of deep (up to 3000 m) oval troughs. The Borisov trough, whose center lies between the villages of Borisovka and Pushchilovka, is characterized by the greatest depth of occurrence of pre-Cretaceous rocks. The troughs filled with Tertiary deposits and tuffaceous formations of the Upper Cretaceous are arranged in a somewhat different plan. The center of the deepest of them is located in the area of the village of Novo-Nikolsk. From the village of Novo-Nikolsk the trough extends in a northeastern direction to the settlement of Vozdvizhenka.

The Cenozoic deposits are represented by two complexes: the Lower Cenozoic (Paleogene, Lower and Middle Miocene) and the Upper Cenozoic (Upper Miocene, Pliocene, and Anthropogene). On the Cretaceous deposits they lie with a sharp angular unconformity.

Oil-prospecting work in the territory of the Suifun synclinal zone was begun in 1959 on the recommendation of a meeting of petroleum geologists held in Vladivostok. Over the course of three years, the Suifun oil-prospecting party drilled 17 wells in various parts of the zone, with a total footage of 13,929 m and a greatest depth of 1250 m. Simultaneously with the drilling work, seismic profiling was carried out by the KMTP and MOV methods, as well as gravity surveying. As a result of the work carried out, in general terms it was determined

structure of the zone and penetrated the section of sedimentary rocks down to and including the Lower Cretaceous.

During the drilling of the latitudinal profile of wells No. 2, 4, 10, and 1, an anticlinal structure was identified. It was not possible, with the work of those years, to determine its structure and trace it over the area. A gravimetric survey in the area of the village of Borisovka revealed an anomaly of gravity. Relative to the structure established by drilling, this anomaly is displaced in a southwesterly direction. On seismic profiles laid across the presumed anticlinal uplift, an absence of reflecting surfaces was noted. On the flanks, a gentle dip of a conditional seismic horizon, drawn along the boundary of the Upper and Lower Cretaceous, was observed. The thickness of the Mesozoic deposits within the Suyfun synclinal zone was determined as 4000–4500 m.

On the basis of MOV data, it was assumed that in the area of the village of Borisovka there is an anticlinal uplift of the horst type. Well 19, at a depth of 476 m, penetrated a bed of conglomerates taken as the basal horizon of the Suchan Formation, dated as Lower Cretaceous. This fact was regarded as proof of the conclusion drawn from MOV data. It was assumed that the horst is bounded on the west and east by faults.

Recently, new data have been obtained from a number of wells, requiring a revision of earlier ideas about the structure of the Borisov uplift. In particular, well 54, drilled as a twin of well 19, entered the deposits of the Suchan Formation and at present, with a bottom-hole depth of 1070 m, has not passed out of them. Core samples from this well contain plant impressions which, according to the determination of S. A. Bayula (Primorye Geological Administration), are common for Lower Cretaceous deposits of Primorye. Thus, the conclusion was reached that the Borisov uplift is not a horst, but most likely a simple anticline (Fig. 1).

A number of profiles, carried out at intervals of 1 km, have traced the strike of the Borisov structure to the northeast for more than 7 km. Drilling was carried out down to the top of the Suchan Formation. As a result, it was established that the anticline has an asymmetric form in cross section. Its western limb is steeper (up to 30°), while the eastern one is gentler (15–20°). The form of the structure is elongated. Well 56 revealed a thrust. Here the Lower Cretaceous deposits of the Suchan Formation are thrust over the formation of mottled sandstones of Upper Cretaceous age and are separated from them by a crushing zone represented by brecciated sandstones cemented with kneaded dark-gray clay. The measured thickness of this zone is 7–8 m; its bedding elements have not been established. Similar crushing zones with thicknesses from 0.5 to 8.0 m were also observed in other wells; however, the emplacement of older rocks over younger ones was nowhere observed except in this well.

The Borisov anticline is of great interest from the standpoint of its possible oil-bearing capacity. Its prospects are enhanced by the fact that bitumen occurrences were discovered here during drilling. Thus, in 1964, vaseline-like bitumens were encountered in wells 54, 56, 60, and 61. The character of the bitumen occurrences remains one and the same: they fill fractures and cavities in the sandstones of the Suchan and Frentsevka formations, or they stain yellow the calcite filling the fractures. Thus, in well 54, in the interval 762–765 m, smears of viscous bitumen were found along fractures in quartz–feldspar sandstones of the Suchan Formation. The bitumen had a light-brown color and, under the luminescope, gave a yellow luminescence characteristic of tarred-

... bitumens. Calcite developed along these fractures was stained yellowish and luminesced with a yellow color.

In well 60, a bitumen show was found in the interval 280–292 m in the deposits of the Frasnian suite. It was expressed in the fact that, when the core was split, a yellow-brown coating adhering to the fingers was visible in the fractures, and also in the fact that in places the calcite filling the fractures was stained brown. Samples with bitumen and stained calcite were examined under a luminescence microscope and gave luminescence of various colors—from reddish-brown, characteristic of resinous-asphaltic bitumen, to yellowish-white and whitish-blue, characteristic of oily bitumens. Bitumen shows of a similar character were also present in other wells.

Fig. 1. Structural map of the Borisov anticlinal uplift, constructed from geophysical survey and structural drilling data

At present, deep exploratory drilling has already begun on the Borisov anticlinal uplift.