Scholarly record

APPLICATION OF BOREHOLE ELECTRICAL IMAGING TO THE STUDY OF SARMATIAN DEPOSITS FROM NORTH-WESTERN MOLDAVIAN PLATFORM (ROMANIA)

Bogdan Mihai Niculescu

Abstract

Integrating conventional well log interpretation with high-resolution borehole electrical image analysis enables a complex understanding of the geological formations intercepted by exploration wells. Whereas conventional geophysical well logs allow the determination of the main reservoir parameters and the fluids distribution, borehole imaging serves as a valuable tool in sedimentary features identification and interpretation, regional dip determination, structural and tectonic elements identification, as well as the analysis of borehole failures (such as breakouts and natural or drilling-induced fractures) related to tectonic stress in a particular area. In addition to conventional well log interpretation, we have carried out a detailed analysis of the borehole electrical imaging data for a gas exploration well drilled in Sarmatian (late Middle Miocene) deltaic deposits from north-western Moldavian Platform, Romania. 1100 borehole features were picked and interpreted across a 196 m interval (386 ? 582 m depth) covered by the borehole image. The number of picks provided a satisfactory statistical dataset to carry out a dip analysis, faults and fractures characterization and present day stress analysis. The features identified on the borehole image include bedding and soft-sediment deformations (in shales), cross-bedding (in sands and silty sands), faults, open fractures, borehole breakouts and unconformable surfaces. A statistical analysis of 775 low-angle planar features picked in shale intervals and representing bedding revealed two dominant dip directions towards SSW (200 ? 220 degrees dip azimuth, 3.1 degrees dip angle modal value) and SSE (150 degrees dip azimuth, 7.2 degrees dip angle modal value), with subordinate preferential dip directions towards ENE or east (60 ? 100 degrees azimuth). The dominant directions are consistent with the general structure of the Moldavian Platform, which dips from NE to SW and, also, towards south or SSE. Two high-angle faults (55 ? 58 degrees formation dip angles) were recognized on the borehole image and they might have provided pathways for biogenic gas migration from deeper levels up to shallower Sarmatian sand reservoirs. Borehole breakouts were found on various intervals and they show a counterclockwise rotation of their azimuth (from 155 to 116 degrees) along with increasing depth, attributable to localized stress field perturbations resulted from faulting.

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DOI resolver: doi.org/10.5593/sgem2019/1.2/s06.081

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