Well Logging Data Interpretation in Oil and Gas Source Rock Sections Based on Complex Petrophysical and Geochemical Analysis Results
Authors: Khisamov Rais , Skibitskaya Natalya , Kovalenko Kazimir , Bazarevskaya Venera , Samokhvalov Nikita , Bolshakov Mikhail , Kuzmin Vladimir , Marutyan Oleg , Navrotskiy Oleg , Pugo Tatyana 
Published: October 2018

The capability to define the content and degree of catagenetic maturity of organic matter (OM) from well logging data and advanced petrophysical and geochemical studies is challenge in petroleum exploration and production.

To date, quite reasonably understood interconnected patterns of catagenetic organic matter transformation in carbonate rock-forming organic polymer matrix (OPM) from gas-condensate, oil-gas-condensate and oil reservoirs. These conversion processes (chemical, structural, physicochemical), polymerization, OM polycondensation within the primary (lithogenetic) crystals are resulted in the loss of ionic OPM crystal connections between organic anions and inorganic cations (calcium, magnesium etc.). It leads to crystals degradation, micritization and to the output of a separate OM polycondensed phase called kerogen. Processes of chemical degradation of kerogen (which fills subcapillary pores of micritizied matrix) to asphaltenes, heavy resins (primary reservoir oil components) and then to the light resins, oils and liquid petroleum hydrocarbons initiate and accompany further recrystallization processes such as transformation of micritizied subcapillary matrix to a coarse-grained large pore matrix. In other words, in carbonate oil and gas filled formations the processes of oil generation and sequential conversion of “immature" high-bituminous heavy oil to light traditional “mature" oil are passed simultaneously and are interrelated with the processes of recrystallization of the micritizied carbonate matrix to coarse-grained large-pore matrix. As a result, light and “matured" to liquid hydrocarbons syngenetic oil occur in a large porous, permeable and even highly-permeable reservoir without displacement in the volume of the transforming matrix.

The study of the regularities of the interrelated processes of the catagenetic transformation of OM in the terrigenous and terrigenous-carbonate oil-and-gas source matrix requires more attention in connection with the need to develop hard-to-recover hydrocarbon reserves associated with the oil and gas deposits of Western Siberia, Caspian, North Caucasian and Volga-Urals provinces.

A set of laboratory core investigations was developed for the purpose of detailed study of both carbonate and siliclastic rocks. Core samples were studied with petrophysical, geochemical, electron-microscopic and physicochemical techniques. Based on the developed approaches relevant correlations between core and well logging parameters were obtained along with designed effective core referencing to logging data.

To determine the nature of saturation of pore volumes with reservoir fluids and bituminous components from well logging data a special technique was developed and a corresponding chart was created.

The basis of the technique forms data and correlation relationships between relative resistivity factor from well logging and mass content of high molecular bitumen components, relationship between mass content of high molecular components and open porosity, relationship between relative resistivity factor and open porosity and oil and gas bulk saturation.

The carried out investigations made it possible to establish the relationship between relative resistivity factor and weight concentrations of high molecular bitumen components at the corresponding stages of oil generation.

The number of stages of oil generation and the amount of produced hydrocarbons in oil and gas reservoir volume is defined by the degree of rock matrix heterogeneity formed in the stages of sedimentogenesis and diagenesis, the composition of its organic and inorganic constituents and the nature of the relationship between them.

Results allow declaring that transformation processes in oil and gas reservoir do not occur simultaneously at certain depths and thermobaric conditions, but take place focally at the level of micro- and macro-volumes in the source rock section. Such focal points in the oil and gas carbonate sections are characterized by a regular increase in porosity and permeability from periphery to center and by change in oil composition from heavy, high bituminous to light, liquid, and traditional oil.

As studies have shown, in the studied oil-source terrigenous formations from the Lower Cretaceous to the Lower Triassic deposits an inverted regularity is appeared.

The final result of the studies is a probabilistic forecast of focal zones in a three-dimensional reservoir model along with differential characteristic of rock properties and fluid types as they approach from the periphery to the center of focal points, depending on the degree of development of focal points and on the lithology of the oil-and-gas source formation.