Page 20 - Geology of Carbonate Reservoirs

CHAPTER ONE

INTRODUCTION

The goal of this book is to explain in plain language for the nonspecialist how and
where carbonate rocks form, how they do, or do not, become reservoirs, how to
explore for carbonate reservoirs or aquifers in the subsurface, and how to develop
them once they have been found. The book is organized around a genetic classifi ca-
tion of carbonate porosity and ways it can be employed in exploration and develop-
ment. The genetic categories include three end members — depositional pores,
diagenetic pores, and fractures. Genetic pore categories are linked with geological

processes that created, reduced, or enlarged pores during lithification and burial. In
the end, a chronology of pore origin and evolution is developed to put in the larger

stratigraphic context for identification of reservoir flow units, baffles, and barriers.

Connectivity can be evaluated by determining the range of porosity and permeabil-
ity values for the different pore categories within reservoirs. Connected pore systems
can be correlated stratigraphically to identify reservoir zones that have the highest
combined porosity and permeability and the least resistance to the passage of fl uids.
Such zones are defi ned in this book as reservoir fl ow units somewhat similar to the

definition of Ebanks ( 1987 ; Ebanks et al., 1992 ) but different in that rock units that

impede fl ow are defined as baffles and units that prevent fl ow are defi ned as barriers.

Each end - member reservoir type generally has characteristic pore - scale features
(porosity and permeability) that correspond to petrologic and stratigraphic proper-
ties (borehole - scale features). When the zones with good, fair, and poor connectivity

are identified, the characteristic petrologic and stratigraphic features that corre-
spond with them can become proxies for connectivity . The larger scale features, or
proxies, are generally easier to identify in borehole cores, on wireline log traces, and
in some sequence stratigraphic “ stacking patterns. ” When mode and time of origin
of the proxies are known, geological concepts can be formulated to predict the

Geology of Carbonate Reservoirs: The Identification, Description, and Characterization of Hydrocarbon

Reservoirs in Carbonate Rocks
By Wayne M. Ahr Copyright © 2008 John Wiley & Sons, Inc.
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