Formation evaluation challenges in Pliocene gas-bearing reservoirs from the Romanian Western Black Sea shelf

Bogdan Mihai Niculescu,Gina Andrei

doi:10.1051/e3sconf/20186601004

Abstract

This paper addresses some formation evaluation challenges and petrophysical particularities regarding two gas fields of Early Pliocene age, belonging to the biogenic hydrocarbon system of Western Black Sea Basin - Romanian continental shelf. Although these structures are located at the same depth and only 15 km apart, the wells that intercepted the sands and silts gas-bearing reservoirs indicate an important lateral facies variation and different reservoir qualities. We analyzed and interpreted data from exploration and appraisal wells that targeted these reservoirs, showing that: (1) there is a limited radioactivity contrast between the reservoir and nonreservoir intervals, so a clay volume determination based solely on the gamma ray log is not practical; (2) the reservoirs are characterized by high capillary-bound water contents, leading sometimes to abnormally low resistivity readings; (3) an additional resistivity suppression might be caused by the limited vertical resolution of the electrical logging tools, in the presence of thinly laminated sand - shale intervals; (4) the identification of gas-water contacts based exclusively on pressure gradients may be inaccurate and should be checked against the results of conventional geophysical logs interpretation and of nuclear magnetic resonance logs, for delineating the intervals with bound water or with movable fluids.

Highlights

For oil and gas exploration wells, formation evaluation consists of the analysis and interpretation of geophysical well logging data and other types of subsurface information in order to ascertain if commercially producible hydrocarbons are present and to determine the best means for their recovery
This paper addresses some of the formation evaluation challenges and petrophysical particularities regarding two gas fields of Early Pliocene (Dacian) age, belonging to the biogenic hydrocarbon system of Western Black Sea Basin - Romanian continental shelf
We show and discuss particular issues related to the log interpretation and formation evaluation for these fields, together with the implications of depositional facies variability upon the resource estimates

Summary

Introduction

For oil and gas exploration wells, formation evaluation consists of the analysis and interpretation of geophysical well logging data and other types of subsurface information (e.g. well testing, formation testing and coring) in order to ascertain if commercially producible hydrocarbons are present and to determine the best means for their recovery. They were identified and imaged using 2D and 3D seismic reflection surveys, started in 1992 and carried out until recent years These dome structures are located at the same depth and only 15 km apart, in similar four-way dip closures, the exploration and appraisal wells that intercepted the sands and silts gas-bearing reservoirs show an important lateral facies variation between the two structures and clearly different reservoir qualities. Such variability cannot be detected on the seismic sections (both structures have quasi-identical reflection signatures), but is evident from the openhole geophysical logs responses and the drill cores taken from the reservoir intervals. We show and discuss particular issues related to the log interpretation and formation evaluation for these fields, together with the implications of depositional facies variability upon the resource estimates

Geological settings

Data and methodology

Results and discussion

Conclusions