Page 18 - Geology of Carbonate Reservoirs
P. 18
ABOUT THIS BOOK
To understand carbonate rocks at reservoir scale, one first has to understand them
at pore scale. Carbonate reservoirs are porous and permeable rocks that contain
hydrocarbons. Carbonate porosity includes three end - member genetic categories:
purely depositional pores, purely diagenetic pores, and purely fracture pores. Inter-
mediate types exist, of course, but the point is that there are three main types of
carbonate porosity that represent distinctly different geological processes. Before
one can fully appreciate these differences and be proficient at distinguishing between
the varieties of carbonate reservoir types, one must understand what carbonates are,
how and where they form, and how they become reservoirs. One must understand
the differences between reservoirs, traps, and seals and learn to appreciate that res-
ervoir characterization is the study of rocks plus the fluids they contain. The opera-
tive word is rocks. Carbonate rocks consist of component particles and maybe some
lime mud matrix and cement. The skeletal and nonskeletal particles, along with mud
and cement, hold an enormous amount of information about the depositional and
diagenetic environments that produced the reservoir rock. This book begins with
definitions, with discussions about how, where, and why carbonates are formed and
about how fundamental rock properties are used to create a language for commu-
nicating information about the rocks — carbonate rock classifi cations. Reservoir
porosity and permeability are variables that depend on fundamental rock proper-
ties. The book explores how rock classifications do or do not correspond with
conventional porosity classifications. Reservoirs contain fluids; therefore we
explore reservoir properties such as saturation, wettability, capillarity, and capillary
pressure.
Geophysical (borehole) logs are briefly mentioned because they provide infor-
mation about third - order rock properties. Logs provide important information to
develop static and dynamic reservoir models, to calculate fluid properties such as
saturation and movable oil volumes, to make stratigraphic correlations, and to inter-
pret lithological characteristics in boreholes where no rock samples are available.
xv
