Abstract: Diagenetic Clays as Pore-Lining Minerals in Coalbed Methane Reservoirs

FOWLER, KATHLEEN S., and KEVIN E. NICK

Cleat surfaces from Mary Lee and Black Creek coal seams in the Black Warrior Basin and Fruitland coal from the San Juan Basin show significant amounts of diagenetic quartz, illite, kaolinite, carbonate minerals, barite, gypsum and iron sulfides and sulfates. Scanning electron microscopy was used to identify and describe diagenetic minerals and surface textures observed along permeable cleat surfaces.

SEM-EDS analysis reveals a variety of pore-lining minerals in the permeable cleats of preserved core and mine samples. Surface textures vary from smooth and vitreous to rough and irregular with embedded diagenetic minerals. The most abundant clay mineral is illite which occurs as surface coatings, aggregates, and authigenic crystals embedded in the coal surface. Kaolinite is also abundant and occurs as abraded platelets and loosely attached aggregates packed against steps, as meniscus shapes on smooth fracture faces, and as a thick crust of subhedral to anhedral crystals. Chlorite, the least abundant clay, appears as sheets of small crystals. Sulfate, sulfide and carbonate minerals are present in masses of euhedral crystals or concentrated as thick crusts. Diagenetic minerals often control surface texture which, in turn, impacts the distribution of mobile particles. Migrating coal fines and clays accumulate at surface irregularities such as steps, laminations of interbedded clays or sulfides, and areas of rough surface texture. Their distribution suggests mobility within the cleat system.

Identification of cleat lining materials is important when predicting fluid/rock interactions, therefore the use of scanning electron microscopy to examine cleat surfaces should be a routine part of completion design. Application of this technology when characterizing Mid-Continent coalbed methane reservoirs will minimize the likelihood of formation damage.

AAPG Search and Discovery Article #90944©1997 AAPG Mid-Continent Section Meeting, Oklahoma City, Oklahoma