containing a disproportionally high number of
source rocks (Yallup and Gréselle, this issue). Over
a quarter of the world’s petroleum reserves are
attributed to generation from Cretaceous source
rocks (Klemme and Ulmishek, 1991; Treloar,
2019). Moreover, Cretaceous organically-enriched
sediments now form some of the most important
resource plays in the world, for example, the Vaca
Muerta play in the Neuquén Basin of Argentina
and the Eagle Ford play in southern Texas.
The Cretaceous
World: Unique
Geology and
Rich Petroleum
Endowment
The Cretaceous represents a major spike in the
abundance of proven conventional carbonate
reservoirs (Markello et al., 2008; Treloar, 2019),
as exemplified by the aforementioned rudist
reefs. These reservoirs frequently have high
primary porosities and permeabilies. Additionally,
their reservoir quality was commonly enhanced
by meteoric diagenesis that took place during
exposure, often relating to moderately high-
magnitude eustatically-driven and/or tectonically-
driven sea level falls.
by: David Ray, Mike Simmons, and Frans
van Buchem
Some of the most prolific siliciclastic reservoirs are
Cretaceous in age, including many associated with
major delta systems, such as the Burgan in Arabia
and the Achemov in Western Siberia. Such delta
systems formed in response to hinterland uplift
created by the ongoing geodynamic reorganization of
the Cretaceous, coupled with eustatic sea level fall.
Chalk cliffs at Ringstead Bay, Dorset, England.
THE UNIQUE GEOLOGY OF THE
CRETACEOUS
The Cretaceous (145 to 66 million years ago)
was a distinctive period of Earth’s history. During
this time, the former supercontinent of Pangea
continued to break up, resulting in the creation
of the South Atlantic, the separation of elements
of Gondwana, and the initiation of the closure
of the Tethys Ocean (Figure 1). The climate was
warmer than today, although variable. Marine
ecosystems underwent radical change, including
the proliferation of calcareous microplankton and
nannoplankton, and the rise and demise of rudist
bivalves, a fauna that often dominated tropical
carbonate shelves during the Cretaceous.
The accelerated creation of oceanic crust resulted
in a long-term Late Cretaceous eustatic peak in
sea level (Conrad, 2013), and with calcareous
nannoplankton proliferating in high-productivity
oceans, this led to the widespread deposition
of chalk (Mutterlose et al., 2005). Episodes of
marked short-term eustatic sea level fluctuation
are also reported (Ray et al., 2019), as are carbon
isotopic excursions and a number of biological
extinctions, including the well-known event at the
Cretaceous–Tertiary boundary.
Further notable features of the Cretaceous are
the repeated intervals of marine anoxia, termed
ocean anoxic events (OAEs), which appear, in
part, to have been associated with the eruption
of large igneous provinces (Jenkyns, 2010), in
themselves yet another distinctive feature of the
Cretaceous. Ocean anoxia led to the widespread
deposition of sediments with a high total organic
carbon (TOC) content. The presence of these
sediments alongside organically-enriched
sediments formed by other processes (e.g.
restriction in newly-opening basins, such as the
South Atlantic), results in Cretaceous successions
The unique geology and rich petroleum endowment
of the Cretaceous encompasses a series of events
and trends that can act as powerful predictive tools.
Two stratigraphic charts described below illustrate
the main events and trends that can facilitate
Cretaceous petroleum exploration.
CHART 1: CRETACEOUS SEA LEVEL
CHANGE AND THE INTERPLAY
BETWEEN TECTONICS AND
CLIMATE
The Cretaceous is characterized by a long-term
rise in sea level and progressive flooding and
erosion of the continental shelves. This rise in
sea level is perhaps most famously recognized in
the Zuni Sequence of the North America craton
(Sloss, 1963), but is global in extent, with peak
sea level occurring in the Late Cretaceous. The
precise height of this long-term eustatic peak
is much debated, but is often cited as being in
excess of 200 m above present-day levels (e.g.
Conrad, 2013).
Driving factors contributing to this sea level
rise include changes in mid-ocean ridge length,
spreading rates, oceanic area, sedimentation,
mantle convection, superplumes, large igneous
province emplacement, and ice volume (Müller
et al., 2008; Conrad 2013). The most important
factors are considered to have been those that
controlled the mean age and relative buoyancy
of ocean crust, which acted to displace the
overlying oceans onto the continental shelves.
These factors may have accounted for over
two-thirds of the Cretaceous long-term sea
level rise. In addition, repeated episodes of large
igneous province emplacement on the seafloor
are believed to have elevated eustatic sea level by
a further 80 m by the Late Cretaceous sea level
peak. Accordingly, much of the long-term rise in
Cretaceous sea level may be accounted for by
elevated crustal production rates in response to
Figure 1> A geodynamic reconstruction of the Earth during the mid-Cretaceous, based on the Neftex ® Plate
Model.
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