Low Heat Flow at Shallow Depth Intervals: Case Studies from Belarus

The territory of Belarus belongs to the western part of the Precambrian East European Platform. Its heat flow pattern is representing by alternating low and high heat flow anomalies. An overwhelming majority of heat flow determinations and in general of geothermal observations in Belarus were fulfilled in boreholes finished in the platform cover. Within the Belarusian Anteclise, Orsha Depression, western slope of the Voronezh Anteclise their bottom holes are typically within the zone of active water exchange, where the groundwater circulation sufficiently influences on recorded thermograms. For instance, observed heat flow density for a number of studied boreholes is low and ranges on average from 15–20 until 35–40 mW/m2 within the Orsha Depression. In a number of studied holes in the northern part of the structure, its values are surprisingly low. They are observed within upper horizons of the zone of active water exchange with pronounced groundwater circulation. Permeable rocks within the geologic section comprise the platform cover with a number of freshwater intervals. Their base is spread here up to depths of 150–250 m. The most of heat flow observations within this area were studied in boreholes which depths is only 200–300 m, sometimes less, as deeper wells are seldom within this geologic structure. Groundwater circulation within loose sediments cools them, most of thermograms here have a concaved shape to the depth axis. As a rule, heat flow values are sufficiently lower in a number of intervals in boreholes finished in the freshwater zone, relatively to the heat flow observed within deeper horizons of the platform cover. In some of studied boreholes, the observed heat flow is as low as 5–15 mW/m2. In most cases it has a tendency to stabilise only at intervals deeper than 600–800 m. It is the main reason for observed low heat flow zones.

Tectonophysical approach to the analysis of geological and geophysical data on gas-condensate deposits with the complex platform cover

The article presents the results of the tectonophysical approach to the analysis of stress fields and the structure of gas–condensate deposits with the complex platform cover. The discussed case is the Kovykta license area (LA) in Eastern Siberia, Russia. In the upper part of the cross section, the network of fault zones was identified from the relief lineaments and structural data. The dynamic conditions for faulting (compression, extension, and strike-slip) were reconstructed by the paragenetic analysis. The state of crustal stresses in the study area was studied by tectonophysical modeling using gelatin as an optically active material. The applied method was successful in distinguishing between the zones of faults in the platform cover, which differ in the degree of their activity in the specified stress fields. The lower part of the cross section in the NE segment of the Kovykta LA is considered as an example of the tectonophysical interpretation of the electrical and seismic survey data in order to identify the fault zones and reconstruct the corresponding stress fields. Based on the synthesis of the analyzed data, it is revealed that the deposits like the Kovykta gas condensate field (GCF) show the zone-block structure of the platform cover formed under the influence of several stress fields closely associated with the stages of tectogenesis in the adjacent mobile belts. The next objective is to enhance the tectonophysical approach in order to develop a hierarchical model of the GCF zone-block structure, which details need to be known for improving the prediction of sites with the complicated stress-strain state of rocks and mitigating the risks associated with drilling exploration and production wells.

STRUCTURE OF THE EARTH’S CRUST ACCORDING TO GEOPHYSICAL DATA ALONG CHEGEM PROFILE (NORTH CAUCASUS)

Несколькими организациями в последние десятилетия были выполнены региональные геофизические исследования различными методами в пределах Северного Кавказа. Вдоль Чегемского профиля (перевал Китлод – г. Буденновск) впервые к северу от зоны сочленения структур Большого Кавказа со Скифской плитой получены две чёткие субгоризонтальные сейсмические границы, разделяющие, вероятно, платформенный чехол, переходный комплекс и консолидированный фундамент. Платформенный чехол, представленный палеозойскими отложениями большой мощности, по аналогии со многими территориями мира, где уже обнаружены и разведаны уникальные месторождения нефти, включая шесть залежей углеводородов, в числе которых и газоконденсатное Кармалиновское месторождение в зоне Передового хребта, может оказаться перспективным на нефтегазовые структуры. Результаты глубинных геофизических работ последних лет могут скорректировать направление поисков углеводородов в регионе. Известные в этом районе Предкавказья восточнее Чегемского профиля нефтегазоносные структуры осадочного чехла приурочены к выделяемым по геофизике поднятиям в консолидированной коре или к их краевым частям. Это даёт основание ожидать наличие углеводородов, вероятно, в материнских палеозойских образованиях Чернолесской впадины, а также соответственно новые перспективные участки и в осадочном чехле по её обрамлению. Several organizations in recent decades have carried out regional geophysical studies by various methods within the North Caucasus. Along the Chegem profile (the Kitlod Pass – town of Budennovsk), for the first time to the north of the junction zone of the Greater Caucasus structures with the Scythian plate, two distinct subhorizontal seismic boundaries, probably sharing a platform cover, a transitional complex and a consolidated foundation have been obtained. The platform cover, represented by large Paleozoic deposits, by analogy with many areas of the world where unique oil deposits, including six hydrocarbon deposits, and the gas condensate Karmalinovskoye deposit in the zone of the Forward Ridge, have already been discovered and explored, may prove promising for oil and gas structures. The results of deep geophysical studies of recent years can correct the direction of hydrocarbon exploration in the region. The oil-and-gas-bearing structures of the sedimentary cover, known in this region of the Ciscaucasia to the east of the Chegem profile, are confined to uplifts in geophysics in the consolidated crust or to their marginal parts. This gives grounds to expect the presence of hydrocarbons, probably in the parent Paleozoic formations of the Chernoslavskaya depression, and also, respectively, new promising sites in the sedimentary cover along its framing.