Visual Rock Characterization can Benefit Reservoir Analysis

Abstract

Information extracted from visual analysis of rock samples focuses us on the fundamentals of oil and gas exploration and development. These data can be found in the millions of boxes of existing cores and cuttings (Figs. 1–4), as well as during drilling at the wellsite. A significant amount of information can be extracted from cuttings, even those chewed up by a polycrystalline diamond compact (PDC) bit. Cuttings and core description brings out details of reservoir pore systems, depositional environments, facies description, supplements and enhances modern wireline logs, and aids in recognizing by-passed pays. Quantitative description has progressed from thin sections to enhanced imaging techniques. There is still a role for cuttings and core description in this changing technological environment. Grain size, framework, fossils, color and texture distinguish subtle facies changes, subsidence patterns, and regional structures.

Rock description provides a tool to calibrate wireline logs to the rocks for quality assurance, better interpretation and even an early calibration to geophysical properties. The character of the matrix and the accessory minerals in the rock affect wireline logs, e.g. pyrite or sylvite, decreasing the uncertainty in wireline log calculations. Shows from samples and cores can be seen to exist in the rock, highlighting potential pay zones.

Diagenetic changes within the rock are visible in cores as well as cuttings; these changes both create and destroy porosity. The nature and amount of porosity can be qualitatively described, including, not only pore types, but also pore distribution, and the type and amount of cement. The recognition of multiple pore types has resulted in identifying overlooked pay zones, as the finest pores have higher adsorbed water percentage and larger pores will flow hydrocarbons. When dealing with unconventional reservoirs, mineralogy and hardness correlate to brittleness, and fractures and microfractures are clearly evident.

AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015