Page 232 - Geology of Carbonate Reservoirs

DEPOSITIONAL RESERVOIRS 213

Published examples of deep - water carbonate reservoirs seem to focus on debrites
and turbidites, including chalk turbidites in the North Sea. Some authors probably

include rock fall and slump deposits with debrites, and grain - flow deposits may be
included with turbidites. It is almost impossible to distinguish subtle differences in
these kinds of deposits without borehole cores. The following characteristics are
common to debrites and turbidites: (1) they are composed of material derived from
upslope or updip in shallower water; (2) they were transported in directions gener-
ally at high angles to slope breaks; (3) they usually occur as fan - shaped deposits
that may or may not be channelized, although massive debrites may occur as irregu-
lar piles of rubble at the base of slope; (4) true turbidites sometimes exhibit fi ning -
upward “ Bouma sequences, ” while debrites lack organized trends in sedimentary
structures and textures; and (5) porosity is generally interparticle, some of which is
probably inherited from the original deposit. Most writers describe porosity in these
deep - water deposits as having been enhanced by burial diagenetic leaching perhaps
associated with fluid migration that attends hydrocarbon migration (Mazzullo,

2000 ). Porosity in the North Sea chalk turbidites is described as interparticle, and
porosity preservation to depths of as much as 10,000 feet is attributed to overpres-
suring and hydrocarbon displacement of interstitial water (Feazel et al., 1990 ).
In summary, interpreting depositional reservoirs requires interpreting deposi-
tional environments and successions because, by defi nition, diagenesis and fractur-
ing are unimportant. Facies maps become proxies for porosity maps, and reservoir
boundaries are defined by facies boundaries, which may parallel present or anteced-

ent structural highs and lows. Permeability corresponds with porosity. High perme-
ability correlates well with high porosity because high porosity occurs in coarser
grained deposits with larger pores and pore throats. Identifying zones with high
permeability and porosity again depends on examination of cores or cuttings to
identify the facies or depositional successions that consistently exhibit coarse and
well sorted textures. Remember the checklist for diagnosis and interpretation of
depositional reservoirs at the end of Chapter 5 .

8.3.2 Selected Examples of Depositional Reservoirs

It is unlikely that purely depositional porosity exists because diagenesis always
affects depositional texture, mineralogy, and porosity to some degree. Original min-
eralogy and microstructure are rarely preserved intact, especially if grain mineral-
ogy was metastable aragonite or Mg - calcite. Instead, altered constituents, textures,

fabrics, and porosity are the rule. The field examples discussed in the first part of
this section were chosen because (1) they clearly produce from pores in which
depositional attributes strongly dominate; (2) porosity is fabric selective; (3) fabrics
and pore types alike are facies selective; (4) the depositional successions can be

identified among the seven standard successions described in Chapter 5 ; and (5) the
3D anatomy of the reservoirs is related to platform bathymetry so that it can be
predicted with the methods described in Chapter 5 . In sum, those fi ve conditions
make it possible to create a geological concept for exploration and development.
Field examples in the second part of this section produce from depositional porosity
that has been significantly changed by diagenesis (hybrid with recognizable deposi-

tional attributes) but still retains its relationship with lithofacies boundaries and
facies characteristics are still proxies for porosity.