Hydrocarbon pools in Jurassic rocks of northwest Siberia and their geological resources

Abstract

Many studies are dedicated to the assessment of the oil-and-gas (hereafter, petroleum) potential of Jurassic rocks in the northern West Siberian petroliferous basin (PB), but only some of them have emphasized the relation between the most informative geological‐ geochemical criteria and geological reserves [1‐4]. The purpose of this work is the systematization of data on geological reserves of hydrocarbons in Mesozoic rocks of the West Siberian PB to define factors responsible for the formation of deposits of different sizes. To accomplish this task, typification of deposits was carried out according to their initial reserves [5] with correlation of defined types to the regional geological‐geochemical settings. Data on initial reserves in Jurassic rocks are available for 76 deposits in the study region. Depending on the reserve values, they are represented by all four groups (kt): unique (>300 000), large (60000‐300000), medium (15 000‐60 000), and small (<5000‐15000). In terms of the phase state, they are divided into oil (O), gas condensate‐oil (GCO), and gas condensate (GC) pools. Figure 1 demonstrates the distribution of total hydrocarbon reserves in Jurassic and Cretaceous deposits. One can see two practically equivalent groups of deposits. The first group includes small pools, while the second group includes medium, large, and unique deposits. The share of unique deposits is high (approximately 18%). Figure 2 shows the distribution of geological hydrocarbon reserves in the Lower‐Middle and Upper Jurassic petroliferous complexes (PC). Small pools prevail in Lower‐Middle Jurassic rocks (38 deposits). Approximately 74% of them have reserves below 5000 kt. These are mainly oil pools (28 deposits). Five deposits are classed with the medium, large, and unique categories. Large and unique deposits include the Bovanenkovo (GCO) and Novoportov (GCO) fields, while medium deposits are represented by the Malyi Yamal (GCO), Urengoi (O), and Malygino (GC) fields. Among 58 hydrocarbon deposits from Upper Jurassic rocks, pools with low reserves are prevalent. Approximately 58% of deposits have reserves below 5000 kt. These are largely oil deposits (28). Eleven deposits (19%) with medium reserves are represented by oil (7) and gas condensate‐oil (4) pools. The group of large hydrocarbon pools includes the Novogodnee (O) and Kharampur (GCO) deposits. No unique deposits are recorded in the Upper Jurassic PC. The distribution of hydrocarbon reserves in the Jurassic demonstrates logarithmically normal patterns. The most frequent (modal) interval of reserves is 1000‐ 10000 kt. The PCs under consideration show some differences in reserves, their distribution, and hydrocarbon phases. A greater number of petroleum pools have been discovered in the Upper Jurassic complex than in the Lower‐ Middle Jurassic complex (58 versus 38 deposits, respectively). The latter group is characterized by higher total reserves in deposits of all groups; greater