Paper ii

SEDIMENTOLOGICAL ANALYSIS UtILIZING THE CIRCUMFERENTIAL BOREHOLE ACOUSTIC IMAGE

H. Verdur, L. Stinco, and C. Naides

Atlas Wireine Services, Western Atlas International, Inc., Neuquen, Argentina

ABSTRACT

The latest version of the Circumferential Borehole Imaging Log (CBIL) device utilizes a focused, rotating acoustic transducer to generate a graphic image of the entire borehole wall. New techniques for acquiring and processing acoustic images permit  improved log definition and identification of textural characteristics of rocks. It is now possible to observe primary and secondary sedimentary features, as well as natural and drilling-induced fractures. Thus, open-hole logs are correlated with textural attributes of the formation. It is also possible to recognize the predominant directions of postdepositional stresses. Openhole log data and CBIL images are combined to analyze sedimentology in a four-well study of carbonate rocks and interbedded sand, silt, clay sequences deposited in a shallow, littoral marine environment. Acoustic images infer:

• Type of lithologic contacts
• Primary sedimentary structures (parallel laminations, cross strata, irregular strata, and strata morphology)      

·         Secondary sedimentary structures generated by carbonate rock dissolution processes

• Discrimination between natural and induced fractures
• Predominant fracture direction and fracture dimensions              

Textural differences between the members of the carbonate series and the underlying and overlying rocks were clearly determined. Reservoir geometry and potential storage capabilities of the traversed for­mations were estimated and a detailed stratigraphic description permitted development of a facies association scheme.