Abstract

The ‘Cenomanian-Turonian’ or C-T is an often-touted ‘world class’ source rock, globally. It has been claimed that a quarter of the world’s oil endowment originates from source rocks of ‘mid Cretaceous’ age; the true stratigraphic age of the responsible source rocks may be centered on the C-T but actually spans the entire Gallic, beginning in the Aptian and continuing into the lower Senonian. One of our ongoing research goals is to establish more specific ages for, and correlations of, the world’s source rocks using the Acme system (This is Petroleum Systems, 2016; Pepper, 2016; Pepper et al., 2017). An Acme is a time, defined in million years based on the GeoWhen absolute timescale, of maximum organic matter (OM) deposition. An Acme is ideally defined by a log of the Ultimate Expellable Potential (UEP; split into UEO for oil and UEG for gas) across the source bed (Roller and Pepper, 2018). Several Ocean Anoxic Events (OAEs) punctuate this ‘mid Cretaceous interval’ and the C-T boundary coincides with OAE2.

Basinal condensed sections can be difficult places to define Acmes because of continuous OM deposition there. So a broad basin to shelf (including outcrop) investigation is needed to define correlative Acmes. In practice this requires correlation across many lithostratigraphic units. In the Gulf of Mexico, rocks of C-T age sensu stricto contain four Acmes, one below the Mid-Cretaceous Unconformity (MCU) and three above:

Acme 99: in an earliest Cenomanian un-named unit in the paleo-deep water basin as well as in the paleo-shelfal environment of the Del Rio Formation in East Texas;

Acme 97: a late early Cenomanian event identified so far only in the ‘Lower Woodbine Organic Shale’ on the paleo-shelf of the East Texas Basin;

Acme 95: an early late Cenomanian event defined on the northern paleo-shelves of the Maverick and East Texas basins; and

Acme 93: an early Turonian event defined on all the northern paleo-shelves of the basin.

This talk addresses the two most significant Acmes 95 and 93, differentiating them—as far as possible with publicly available data—across the Gulf of Mexico Mega-Basin. In Mexico and Florida, a distinction is not yet possible and so we discuss the two Acmes in combination across the whole Mega-Basin.

Paleobathymetry, estimated from heights of the pre-MCU shelf margins and supported by tectonic subsidence modeling at Deep Sea Drilling Project (DSDP) wells, was governed by strong thermal subsidence with ∼4 km of water in the basin center (Fig. 1). The pre-MCU lower Cenomanian and older shelf margins defined a steep shelf-to basin slope, with a drowned terrace on the western side (Mexico) and locally on the U.S. margin, which was created by back-step of the margin during the Aptian.

Lithofacies variation is determined by the input of course- to fine-grained clastics from a north-easterly shoreline; with Mexico and southern Florida represented by clastic-starved carbonate in paleo-shelfal and, we predict, deepwater environments. Clay content decreases westwards from the San Marcos Arch towards the Maverick Basin and on into Mexico.

The marine Organofacies—A, high sulfur and B, low sulfur—of these source beds follows lithofacies: input of clastic mud into the East Texas and Mississippi salt basins creates a clay-rich, low sulfur, Organofacies B OM system; decreasing clay west of the San Marcos Arch caused a transitional A-B mix in southwestern Texas, becoming a fully carbonate Organofacies A on Mexican shelves. Clastic slope sediment wedges developed off the carbonate margin appear to be OM-lean; condensed section source rocks developed in abyssal environments in the U.S. deepwater were fed by northern clastic input and are clay-rich Organofacies B.

Acme 95 includes the ‘Lower Eagle Ford’ of southwestern Texas and has the highest UEP/UEO/UEG in the basin. In the eastern U.S. Gulf, this Acme is time equivalent to the backstepping upper part of the Lower Tuscaloosa Formation where it is overwhelmed by coarse clastics input. In spite of this, some interbedded OM-rich mudrocks developed here are likely the source for the ‘Deep Tuscaloosa Gas Trend’ expanded wedge in Louisiana.

Acme 93 is the most widespread C-T Acme, since it includes the ‘Upper Eagle Ford Fm’ in southwestern Texas and the Middle Marine Tuscaloosa Fm in the Mississippi Salt Basin. However, its UEP is significantly lower than Acme 95. It is eventually overwhelmed in the later Turonian by deltaic course clastic progradation in the Sub-Clarkesville and Upper Tuscaloosa formations in the East Texas Basin and Mississippi Salt Basin, respectively.

Abyssal source rocks (Acme 93 and 95 undifferentiated) have relatively poor UEP compared to the southwest Texas shelf (Fig. 2), which may reflect poor preservation during long settling times in the deepwater basin center.

Interestingly, the two Acmes occur either side of the C-T boundary (93.5 Ma) and so OAE2 is not a time of maximum organic deposition in the Gulf of Mexico Mega-Basin (Fig. 2).

We used bulk characteristics and biomarkers from rock extracts and produced ‘shale’ oils from Acme 93, 95, and undifferentiated equivalent source beds to confirm the Organofacies patterns assigned from lithofacies. Most commonly available data are GC-derived alkane-nomalized isoprenoid ratios (t!Ps’ parameters Pr|17nor, Ph|18nor and Pr17nor|Ph18nor). Using these and other parameters we were able to see subtly changing Organofacies transitions, both vertically/stratigraphically and laterally/depositionally, within the broad Organofacies belts mapped out from lithofacies.