The Chara depression as a new coal and hydrocarbon region (Northern Transbaikalia)

Abstract

524 Recently information on the close connection of coalbearing and oil and gas formations has grown substantially, suggesting spatial and genetic relations for coal and hydrocarbon basins and deposits. This hypothesis was put forward in the early 1990s by M.V. Golytsyn, N.V. Pronina, and E.Yu. Makarova and has theoretical and considerable practical value: it gives an approach to surveys for the entire complex of hydrocarbons in areas that earlier were regarded as potential for only one type of resources (1). A typical example of such an area is the Kuzbass, which apart from coal is also promising for oil and gas, and some hydrocarbon deposits have already been found. One of the potential territories is the Chara Depression, which is a part of the unique Baikal Rift Zone stretching along the southern fringe of the Sibe� rian Craton for 2000 km. The depression is ascribed to a Baikaltype Cenozoic structure and differs from the wellknown Transbaikal Mesozoic depressions that are mostly coalbearing. The Chara Depression runs for 120 km at a width of 20-30 km and is confined to the voluminous Baikal-Chara fault, which divides the Siberian and Transbaikal microplates (Fig. 1). It is known that the overwhelming majority of the world hydrocarbon provinces gravitate towards continental plate bound� aries superposed with active seismic mobile zones, where earthquake sources are located. All these char� acteristics are typical for the Chara Depression and its surroundings, where tectonic activity is inherited from the Precambrian. Here, hydrocarbons occur as gas-coal deposits, the Apsat and Chitkanda, which are parts of a large Mesozoic basin located in Northern Transbaikalia and ranging (considering only modern outcrops) for more than 130 km from west to east at 30-35 km width. The largest Apsat deposit occupies 100 square km in area and contains over 2.2 gigatons of coal (includ� ing coking coal). The deposit has two coal horizons: the lower one being the main one as thick as 360 m with up to 40 beds of coal. This horizon occurs 900- 1000 m below the upper one and yields 75-81% of coking concentrates from enriched coals. The coal's coking produces 0.66-4.23% of pitch, 0.47-1.05% of benzol, 0.1-0.65% of ammonia, and 8.14-18.2% of coke gas. The volatile yield is 30% (2). The coal methane resources (free, absorbed, and fixed) are estimated by IPCON RAS researchers as up to 160-180 billion cubic meters. The coal beds con� tain 50-55 billion cubic meters of methane with an estimated 25-28 cubic meters of methane per ton of coal. Comparison of the Apsat deposit productivity and its area to those of the sole Chara Depression (Fig. 2) shows that the resources of the latter for free natural gas could be estimated at more than 1 trillion cubic meters (3), considering that differentiate tectonic movements within the existing vast JurassicCreta� ceous coalearing basin here resulted in burial of coaland methanebearing beds over an area of about 800 km 2 . It should be noted that in the eastern part of the Apsat deposit facing the Chara Depression where