Complex of geophisical and geochemical methods for hydrocarbon prospects

Napreev D.V.

Scientific production enterprise of geophysical equipment “Luch”, Novosibirsk, Russia

ndv@mail.ru

On the way of the vertical migration of hydrocarbons from the reservoir to the surface during geological time are formed dispersion halos, within which the violated field stability of some minerals, changing the mobility of some trace elements, which leads to the formation of various anomalies in the material composition of the rocks. These abnormalities include elevated concentrations of hydrocarbons in the soil itself, sediments, water and air, high concentrations of non-hydrocarbon gases such as helium and radon; anomalous iodine content, and changes of pH and redox potential of the medium, mineralogical changes in the form of education of secondary calcite, pyrite, magnetite, uranium salts, modified clay minerals, changes in the concentrations of trace elements [1, 2].

Various investigators proposed their own models of anomalies in the interior of rocks, based on a study of any field, most of them repetitive, and in fine detail - complementary.

Consequence of epigenetic changes in rocks are geophysical anomalies over the deposit in the acoustic, electric, magnetic, radiation, and geothermal fields, which creates conditions for the application of shallow geophysical techniques to search for hydrocarbon deposits.

Processes associated with the migration of hydrocarbons from deposits occur throughout the thickness of rock, ranging from the reservoir up to the surface with varying intensity depending on the depth and features of geological strata. The difficulty for researchers is that the set of anomalies characterizing a particular deposit, may vary even for neighboring deposits.

Since 2008 specialists SPE "Luch" and IPGG RAS explore perspective for oil and gas areas by complex ground-based non-seismic methods. This complex formed on the basis of numerous experimental and methodical works done in previous years, as single method, and with varying degrees of aggregation on the known and explored sites in Khanty-Mansiysk, Tyumen, Tomsk and Novosibirsk regions.

On one of the promising for hydrocarbons areas on north of the Tomsk region was investigated complex methods. The main purpose of this study was to search for oil-laying structures for exploration wells deep drilling. To achieve the goal it was necessary to solve the following problem - division of the sedimentary cover on the geoelectric properties, mapping the surface of the Paleozoic base, and search for direct signs of the presence of hydrocarbons at depth.

Within the area of research on a grid of old seismic lines, the following activities: geoelectrical methods of Near-Field Transient Electromagnetic (TEM) Sounding and Magnetotelluric (MT) Sounding, sampling for the investigation in vitro methods of chromatography, determination of pH, redox potential (Eh), magnetic susceptibility, gamma-ray spectrometry. In addition, for the first time an assessment of the polarization properties of the Induced Polarization (IP) Fast Processes Method.

By the electrical methods was detected roof of the Paleozoic rocks, is not allocated by seismic method, set the overall structural plan of the area, and marked anticlinal uplift in the sedimentary complex. A detailed analysis of electrical data, found an increase of the resistivity above positive structures. The nature of this resistance increase in the thickness of rocks above the oil deposit, probably due to the number of geochemical transformations under the influence of the hydrocarbon fluid.

Have been analyzed the distribution of geochemical fields of the area in terms of confinement of geochemical anomalies to the selected structural elements of the section. Were allocated areas of high hydrocarbon content in the soil, the boundary of this area was carried out on the content of benzene and toluene, 0.4 mg/m3, which is in 20 times higher than background values. According to the measurements of induced polarization (IP) on the clay samples in the study area were contoured anomalies of polarizability> 4%. In complex data analysis took into account the anomalies of the redox potential with values ​​<350 mV, and the contours of the anomalies of the magnetic susceptibility of æ <240·10^-5.

As a study result show that the projection to the surface slope areas of positive structures, allocated by TEM and MT sounding, are associated with anomalies of high hydrocarbons concentration in quaternary clays, accompanied by reduced values ​​of the redox potential of clays and their high polarizability and low magnetic susceptibility. This specified set of attributes defined as interpretative criteria to identify sites that are promising for hydrocarbons. Subsequently, within one of anomalous zones by deep drilling was opened oil-bearing collector in the Jurassic sediments.

Complex methods continue to evolve and be supplemented. In subsequent studies already mentioned complex was supplemented by observations of the gravitational field of the earth, measurements of radon emanation, iodine and metal ions. This additional information will facilitated the construction of predictive model because allowed take into account tectonic features of the work area.

References:

1. Schumacher D., Hydrocarbon-induced alteration of soils and sediments, in D. Schumacher and M. A. Abrams, eds., Hydrocarbon migration and its near surface expression: AAPG Memoir 66, p. 71–89, 1996.

2. Saunders D.F., Burson K.R., Thompson C.K. Model for hydrocarbon microseepage and related near-surface alteration // AAPG Bulletin vol.83, p.170-185, 1999