Hydrogeological Conditions for Drainage of Peat-Covered Oil- and Gas-Bearing Areas of Tomsk Oblast

HYDROGEOLOGICAL CONDITIONS FOR DRAINAGE OF PEAT-RICH OIL- AND GAS-BEARING AREAS IN TOMSK REGION

N. M. RASSKAZOV, P. A. UDODOV, T. Ya. EMEL’YANOVA, A. D. NAZAROV

(Submitted by the scientific seminar of the Department of Hydrogeology and Engineering Geology and the Problem Geological Laboratory of TPI)

A large part of the oil- and gas-bearing territories of Tomsk Region is swampy and peat-covered. In this connection, characterization of the conditions for their drainage is highly relevant. Hydrogeological and hydrogeochemical studies carried out by staff of the Department of Hydrogeology and Engineering Geology of TPI jointly with the institute “Giprotorfrazvedka,” NTGU, and other organizations in the swampy districts of Tomsk Region made it possible to establish the relationship between the composition of groundwater, loose rocks, and peat deposits. It was found that the mineralization of groundwater noticeably decreases northward, along with a general decrease in the clay content of the cover deposits and in the areas of distribution of lowland peat bogs. These features are most clearly visible within watershed areas, where the azonal processes characteristic of river valleys, especially large ones, are not manifested. The combination of the listed changing factors is also accompanied by a decrease, from south to north, in the “salinity” (mainly carbonate content) of clayey rocks. This distinctive natural setting, also characterized by a regular increase in atmospheric precipitation and a decrease in evaporation toward the north, has determined specific features in the formation of a number of peat deposits within this territory. They are manifested mainly both in the accumulation of lowland bogs on regionally developed clayey rocks in the southern part of the region, i.e., in the absence of substantial groundwater feeding, and in the formation of raised-type peat deposits in its northern part on sandy deposits under conditions of inflow of weakly mineralized groundwater. These genetic features of peat bogs were taken into account by us in the process of developing a methodology for hydrogeological investigations at various stages of prospecting for peat deposits. They must also be considered in the development of the latter and in the performance of reclamation works. Drainage measures within peat-covered territories can be successfully carried out only on the basis of their appropriate zoning.

Therefore, the authors have attempted to carry out such zoning, taking into account the geological and hydrogeological features of the territories being characterized.

Among them the following are distinguished: I — relatively easily drained, II — with an average degree of drainage difficulty, and III — difficult-to-drain territories (Fig. 1).

Fig. 1 — Zoning scheme of peat-covered oil- and gas-bearing territories of Tomsk Oblast according to drainage conditions. (Compiled by: N. M. Rasskazov, T. Ya. Emelyanova, A. D. Nazarov, 1969).

Legend.
1 — Relatively easily drained watershed territories. Peat deposits are underlain by a thick sequence of clayey sediments. The feeding of the bogs is mainly atmospheric. Peat thickness is 4.5 m; 2 — watershed territories with a medium degree of drainage difficulty. Peat deposits are underlain by a thick sequence of clayey sediments. The feeding of the bogs is mainly atmospheric; the maximum peat thickness is 10 m; 3 — poorly drained territories; 4 — boundaries of peat-covered territories distinguished by drainage difficulty; 5 — a) poorly drained peat deposits of interfluves. The feeding of the bogs is groundwater and atmospheric; 6—b) watershed poorly drained peat deposits. They are underlain by sediments of small thickness and varied composition: clays, loams, sandy loams, sands. The bogs are fed by atmospheric and slightly mineralized groundwater; 7 — c) poorly drained peat deposits of ancient valleys. The feeding of the bogs is by atmospheric and groundwater. 8 — poorly drained territories (identified tentatively); 9 — boundaries of geomorphological elements; 10 — area of development of predominantly raised peat deposits; 11 — area of development of predominantly raised and transitional peat deposits; 12 — area of development of predominantly raised and lowland peat deposits; 13 — area of development of raised, transitional, and lowland peat deposits; 14 — area of predominance of lowland peat deposits; 15 — boundaries of the distribution of various types of deposits; 16 — oil fields; 17 — gas-oil fields; 18 — gas-condensate fields.

I. Relatively easily drained peat-covered areas occupy the watersheds of the Shegarka, Iksa, Bakchar, Parbig, and Kenga rivers and their tributaries. The peat deposits here are small in size, narrow, and elongated along the watersheds. The average peat thickness is about 2.5 m, and the maximum is 4.5 m. The peat deposits are underlain by a considerable thickness (up to 20 m and more) of clays and heavy dense loams. As a result, the interconnection of bog and groundwater in this territory is very difficult. The feeding of the bogs is mainly atmospheric, and only during periods of flood inundation do surface waters take part in it. This circumstance

along with the low carbonate content of the underlying deposits, leads to the formation of raised and transitional, and more rarely lowland, peat deposits.

II. Peat-covered territories of an average degree of drainage difficulty occupy the central, upper part of the Ob–Irtysh watershed and the watersheds of the upper and middle courses of the Vasyugan, Nyurolka, Chizhapka, Parabel, and Chaya rivers (Fig. 1). Most of the surface here is weakly dissected; the headwaters of rivers are often lost in bogs; the dissection of the surface increases northward. The southern part of this territory is occupied by the continuous peat Vasyugan massif, which extends far beyond its limits. North of it, on the watersheds of the above-named rivers, there are peat deposits of considerable area. The peat thickness ranges from 2 to 5 m. Throughout this territory the peat deposits are underlain by a stratum of dense, viscous clays of considerable thickness. The feeding conditions of the peatlands are approximately the same as in the territories of group I. However, the possibility is not excluded that the bogs are fed by discharging groundwaters. Thus, for example, the elevated content of iron hydroxides in bog waters and the presence of strongly water-saturated loams, as well as the confinement of this area to the junction of various geological structures and geomorphological regions (the Vasyugan Plateau, the Near-Ob Plateau, the Baraba Lowland), make it possible to assume the discharge here of the confining bed and an inflow of deeper groundwaters. Such hydrogeological conditions have led to the formation of lowland bogs in places where groundwaters emerge and in areas where clays with high carbonate content occur. The development of the latter is especially characteristic of the watersheds of the Bakchar, Om, and Shegarka rivers, where the CaCO₃ content reaches 13% and more. On less “saline” rocks, transitional and raised peat deposits are developed.

III. Difficult-to-drain peat-covered territories occupy: a) the valleys of the Ob, Vasyugan, Tym, Ket, and Chulym rivers and their large tributaries; b) the watersheds of the middle and lower Vasyugan, Vakh, Tym, Ket, and Chulym rivers; c) ancient valleys (Fig. 1).

a. In river valleys, floodplain and terrace deposits are distinguished. Floodplain deposits are fed by groundwaters and floodwaters. The feeding of terrace peat deposits is varied. On high terraces, many raised-type peat deposits are apparently fed by atmospheric precipitation and surface waters. On low terraces, groundwater feeding predominates. The peat deposits are underlain by loose deposits of varying composition (clays, loams, sandy loams, sands).

b. Peat deposits on watersheds are underlain by a stratum of clays ranging in thickness from 15 m to 1 m, often by loams and sands. Here, in addition to atmospheric feeding and feeding by surface waters, a significant role is apparently played by the feeding of peat deposits by slightly mineralized groundwaters, especially along the gentle slopes of watersheds closer to the valley of the Ob River and its large tributaries. The greatest peat thickness reaches 10 m.

c. Ancient valleys have been identified by us according to the data of M. P. Nagorsky, as well as A. A. Zemtsov (1966). In these areas, raised, transitional, and lowland peat deposits of varying thickness are developed. The deposits are underlain by loose deposits of varying composition—sands, sandy loams, loams, and clays. As in areas a and b, atmospheric precipitation, surface waters, and groundwaters evidently participate in the feeding of the peat deposits.

The conditions of occurrence and feeding of peat deposits characterized above evidently determine the main features of reclamation measures in the various territories. Thus, for example, in areas

for Group I, and partly Group II, the density of the drainage network should be considerably sparser than within peat bogs of Group III. These same features were taken into account by the authors in developing a methodology for hydrogeological investigations at various stages of studying peat deposits, the general provisions of which are set forth in the instructions of the Ministry of Geology of the RSFSR (1964).

At all stages of investigation, we propose, along with other work, to study the microcomponent composition of peats and peat waters, which will make it possible to obtain additional criteria for identifying difficult-to-drain areas where more deeply occurring groundwaters discharge. At the stage of regional investigations, the study of peat-covered areas can be carried out, for the most part, without drilling special hydrogeological wells. Their minimum number should be provided only at the boundaries of districts I and II with III, and also within those parts of the second district where discharges of groundwater are expected. It is also advisable to drill individual wells up to 20 m deep in areas of ancient valleys (III “c”).

During preliminary exploration of peat deposits in the first and second districts, the distances between hydrogeological wells along a profile should be taken as the minimum (6 km), and between profiles as 8 km. Reducing these distances by half, i.e., to a 3 × 4 km grid, is advisable only at the boundaries with the third district and on lowland deposits of the second district. In the course of studying peat bogs in the territory of the third district, the smallest distances specified in the instructions should be provided, i.e., wells should be arranged on a 1 × 2 km grid. Similar recommendations, taking into account the different conditions of peat-covered territories identified according to the zoning scheme (Fig. 1), have also been developed by us for the detailed-exploration stage.

Thus, proper accounting for the hydrogeological features of peat-covered areas makes it possible not only to study their drainage conditions in a purposeful manner, but also to save considerable funds by reducing the number of costly hydrogeological wells in the corresponding areas.

LITERATURE

1. A. A. Zemtsov. Geographic position and relief. In: Natural Resources of Tomsk Oblast. TSU Publishing House, Tomsk, 1966.

2. Technical Specifications for the Exploration of Peat Deposits. Gostoptekhizdat, Moscow, 1964.