Diagenetic Clays as Pore-Lining Minerals in Coalbed Methane Reservoirs

Kathleen S. Fowler 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. SEM, XRD, thin section and reflected light microscopy analyses were used to identify and describe diagenetic minerals and surface textures observed along permeable cleat surfaces.

SEM-EDS analysis reveals a variety of pore-lining diagenetic minerals with complex crystal morphologies in permeable cleats of preserved core and mine samples. Surface textures were varied from smooth and vitreous, dull and pitted, to rough and irregular with imbedded diagenetic minerals, often clays or sulfides. Illite is the most abundant clay and occurs as surface coatings, aggregates, authigenic crystals embedded in the coal surface, or oriented subparallel to the fracture face. 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 anhedral crystals. Chlorite, the least abundant clay, appears as sheets of small crystals. Locally abundant sulfate, sulfide and carbonate minerals are present in masses of euhedral crystals or concentrated as thick crusts. Surface irregularities sometimes control the distribution of diagenetic minerals. Coal fines of unambiguous internal origin and masses of clays are often concentrated at surface irregularities such as steps, laminations of interbedded clays, or sulfides and coal and rough areas of fractures. Their distribution suggests mobility within fractures.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California