Investigation on mechanism of low-resistivity oil reservoir in Minghuazhen and Guantao Formation of Nanpu Sag

Abstract

Per the resistivity wireline log interpretation, some reservoirs in Nanpu sag have water saturation larger than 50%, however, per oil production testing, those reservoirs have low water production rate. Via FEI QEMSCAN system and FEI scanning electron microscope, the mineral composition, content, distribution and texture of samples from low-resistivity reservoirs in Minghuazhen and Guantao Formation of Nanpu sag have been analyzed. Results show that, the studied samples all have high total clay content, with average volume percentage being 11.62%–15.43%; The main clay minerals of the studied samples are chlorite (average volume percentage 23–35%), illite-smectite mixed layer (average volume percentage 26–31%), and illite (average volume percentage 23–30%). Additional conductivity and absorption of water molecule by these clay minerals are important reasons for the low resistivity wireline response of the studies reservoirs. Pyrite content of the studied samples is 0.08%–3.99% in average volume percentage, and it is highly enriched in several samples, which will increase the electrical conductivity of rock grains of those samples. The back-scattered electron images indicate that the original inter-grain pore with pore diameter being microns contributes the most porosity. However, the clay minerals which broadly, homogeneously and dispersively distribute in samples, as matrix, or part of rock debris, or cement, have effectively enhanced the cation exchange capability of reservoirs, and have created large amount of nano-pores. These nano pores will prominently increase the bound water saturation of reservoirs.