Chamosite: Critical Ingredient in Diagenetic Differentiation of Sandstone Reservoirs

Zuhair Al-Shaieb, Mark Lynch

Chamosite, or iron-rich chlorite, is well documented in sedimentary ironstones and generally agreed to have formed in a particular shallow marine environment with specific geochemical and sedimentological controls. However, chamosite is not restricted to ironstones. For example, chamosite is a common constituent in several productive Pennsylvanian sandstones such as the Spiro and Sells sandstones in the Arkoma basin and Springer sandstones in the Anadarko basin.

Chamosite is a penecontemporaneous to early diagenetic mineral that occurs in several distinct morphologies, including coated grains comprised of concentric laminae around detrital nuclei, granules/nodules, thick pore coatings, and pseudomorphous replacement of bioclastic debris. Under plane-polarized light, chamosite varies in color from light green to light brown. X-ray diffraction analysis shows that the 14A°-basal-spacing variety is very common; however, a 7A°-chlorite (berthierine) is also present in lesser quantity.

The presence of chamosite in sandstone is very significant in determining both depositional environment and postdepositional diagenetic history, including reservoir preservation. In the Pennsylvanian sandstones studied, chamosite-rich facies exhibit very distinct diagenetic patterns compared to other facies with less clay. Preservation of primary porosity is the common denominator in all chamositic sandstones, whereas quartz overgrowths and/or other types of cements tend to occlude the pore space in the cleaner facies. Thus, differentiation of these Pennsylvanian sandstones into reservoir-quality and tightly cemented types is directly related to the presence of chamosite.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.