Abstract: Interpretation of Depositional Environments, Porosity Relations, and Diagenesis from Analysis of Carbonate Well-Cutting Samples

A. D. Jacka, T. L. Gray

Well-cutting samples constitute a vast reservoir of valuable data that mainly has been utilized to prepare lithologic sample logs. Because carbonate sediments are predominantly of organic origin, and because they so sensitively record diagenetic effects, much economically significant information may be derived from a comprehensive analysis of the cuttings. Principal analytic techniques include examination with a binocular microscope and petrographic analysis of thin sections. Because of general availability, cuttings may be utilized to delineate local or regional relations of carbonate lithofacies, biofacies, and depositional environments. Knowledge of the mineralogy and petrography of fossils is requisite for interpreting biofacies, depositional environments, and diagene is.

From an economic viewpoint the most significant information that may be derived from the study of carbonate cuttings concerns porosity relations. The diagenetic overprint, superimposed on lithofacies, biofacies, and depositional environments, always exerts the dominant control on development, occlusion, and preservation of porosity in carbonate reservoirs. Diagenetic factors that importantly influence porosity relations in carbonate rocks and which may be determined or inferred from the analysis of cuttings include: (1) early post-depositional diagenesis within the marine environments, (2) dolomitization, and (3) a complex diagenetic syndrome associated with subaerial exposure and effects of meteoric, vadose-phreatic ground-water dynamics.

In interpreting and attempting to predict subsurface porosity relations it is just as important to comprehend lack or occlusion of porosity in nonproductive wells or intervals as it is to understand formation and retention of porosity in reservoirs. Cored intervals commonly include only pay zones where porosity was preserved. Cuttings thus may constitute the only available data for analyzing lack of porosity or porosity occlusion.

Nearly as much significant information can be obtained from good cuttings as from cores. Even complex diagenetic sequences can be reconstructed from study of thin sections prepared from cuttings. Porosity relations can be interpreted reliably from integrated analyses of logs and cuttings, and subsurface predictability may be increased greatly. Cuttings samples thus represent a large, virtually untapped source of valuable data that may be used effectively in exploration for and exploitation of carbonate reservoirs.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA