Page 66 - Petrophysics 2E

40 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

generally exceeds the volume of sediments entering the basin, and hence
the depth of the sea increases. As the sea advances over the land surface,
the depositional facies also migrate inland, creating a shallow, low-energy
environment along the shore that tends to accumulate fine-grained
particles. The fine-grained sediments have low permeability and are
potential petroleum source rocks rather than reservoirs [9].
During a regressive phase in the formation of a basin, the basin
becomes shallower and the depositional facies migrate seaward into
a high-energy environment. A regressive sequence may develop because
the supply of sediments is greater than the amount accumulating in
the basin that can be removed by the available energy. This occurs in
river deltas where the delta is growing because the supply of sediments
to the delta is greater than the amount of sediments being removed from
the area by sea currents and waves. Thus, one of two elements may be
active: (1) the sea level may be decreasing, or (2) the sediment supply
may exceed the capacity for removal and redistribution. The sediments
accumulating during the regressive phase tend to be coarse-grained
because of the higher energy level in the depositional basin during
this period. The rocks of this sequence, therefore, have relatively high
permeabilities and are potential reservoirs layed down on top of potential
source rocks deposited during the transgressive phase.
The transgressive-regressive stages tend to accumulate sequences
of sediments that are either shalehand or shale/carbonate-evaporite.
The carbonate-evaporite sequences are associated with some, but not
all, of the transgressive phases resulting in periodic accumulations of
carbonate-evaporite lithologies. The low-energy environment of the
shallow shelves provides opportunities for development of abundant
shellfish whose shells become beds of limestone. Calcium and magne-
sium tend to precipitate from the shallow seas resulting in depositions of
limestone (CaC03) and dolomite [CaMg(C03)~]. Porosity is developed
by dolomitization, chemical leaching by percolating waters (solution
porosity), and mechanical fissuring from structural movements leading
to jointing and vertical cracks. Carbonates also are deposited as reefs at
the edge of continental shelves and along the continental slope.

ACCUMULATION OF SEDIMENTS

The accumulation of sediments in a given area depends on equilibrium
between the energy of the environment and the inertia of sedimentary
particles. For example, sediments transported to the mouth of a river
may be moved by waves and currents to another location where the
environmental energy is not high enough to move the particles. This
is the concept of base level [9]. Sediments of a given size and density