Page 138 - The Petroleum System From Source to Trap

132 Morse

PROGRADATION -
B LAGOON WASHOVER FAN BEACH-DUNE RIDGES
A SP RE!'ii�TIV!T'Y

AllUVIAL
SI\NDSTONE.
MUDSTONE.
COAL
PROGRADING
NEAASHORE
... ARINE
SANDSTONE c
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MARINE /
SHALE ANO /
/
Sll TSTONE
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Figure 6.6. Shallow marine reseirs include shoreface sands associated with barrier islands and tidal channel deltaic and
offshore bar sands. With fluctuating sea levels, these sands may form laterally extensive blanket sands that can become
stacked (after McCubbin, 982 1 ; Reineck and Singh, 1 980 ). (A) Electric log of prograding shallow marine to coasl marsh
sequence. (B) Cross secton of a barrier island with prograding shoreface and beach-dune facies. (C) Block diagram of
marginal to shallow marine depositional facies.
Both transgressive and regressive sequences are identi­ canyon onto a basin plain. Walker (1979) and Howell
fied in these stacked sequences. Although stratigraphic and Normark (1982) have described these processes and
traps are particularly common for offshore bars, such as the deposits that are formed. The upper fan (Figure 6.7)
in the Cretaceous fields of northeastern Wyoming, few contains the coarsest grained sediments, including large
fields of this facies are listed on this table. Most fields in blocks from the shelf or that have broken loose from the
Table 6.5 were formed by either tilted blocks and uncon­ canyon walls; these are grain-supported conglomerates,
formity pinchouts or by rollover anticlines. Few sedi­ matrix-supported debris flow conglomerates, and coarse­
mentologic details have been published about the grained turbidites. This coarse-grained material,
shallow marine reservoirs in the supergiant fields of deposited primay in the upper channel valley, is thick
eastern Europe (Halbouty, 1980b; Klemme, Chapter 3, and does not extend laterally for more than a few kilo­
this volume), but this should change when oil and gas meters. Finer suspended material may be carried to the
companies from outside eastern Europe become more laterally extensive levees or interchannel areas.
active in developing the larger fields. Deposits on the mid-fan or suprafan lobe are com­
posed of turbidites, channelized distributary sands, and
increasingly larger deposits of interchannel, finer grained
DEEP MARINE RESERVOIRS sediments. The turbidites are normally graded and are
likely to contain sand beds 0.25-2.5 m thick that are
Deep water (marine) reservoir rocks are deposited in organized into subdivisions of the Bouma sequence
subaqueous fans that occur in both marine and lacustrine (Bouma, 1962) and that extend laterally with little change
settings and include many gravity-driven depositional in thickness. The channels shift by avulsion and ulti­
processes. Deep water fans receive shallow water or shelf mately cover much of the fan lobe and form a series of
sediment from feeders on the slope, such as an old river fining-upward sandstone beds with sand aggregates of
canyon inherited from a lowstand of sea level. These up to 80 m. Sand bed thickness and relative amount of
feeders are relatively steep and downcutting and act as sand decreases with increasing distance outward from
conduits for all reworked sediment that is deposited on the head of the f a n.
the fan. Depositional mechanisms include turbidity flow, The outer fan begins where channels no longer form,
debris flow, free fall or cascade, and traction flow. The yet lobe building continues by deposition of silts and
first three can occur very quickly and with enough clays from suspension in waning turbidity currents.
energy to transport silt up to 100 km from the base of the
Sandstone beds are thin and rare here, being transported