Even though time remains the basic parameter used in basin modeling to predict source rock maturation and hydrocarbon migration, we still lack a technique for direct dating of expulsion that can be applied to petroleum or the source rock. It’s a challenge that geologists and geochemists have wrestled with for many years, because that knowledge would be a major step forward in improving our evaluation of petroleum systems and steering our exploration strategies in the right direction.
That was the starting point for our research into whether uranium-lead (U-Pb) geochronological dating could be applied to diagenetic cements. Thanks to a breakthrough method developed by Total in collaboration with the CEREGE geochronology laboratory (Aix-en-Provence), we can now obtain a precise absolute age independently of basin modeling.
Teaching Diagenetic Cements to Speak
As diagenetic cements form during reservoir burial, they record two vital pieces of information: the paleotemperature in the reservoir and the possible presence of hydrocarbons in the pores. In other words, those cements contain essential data for recreating the history of how a petroleum system has originated and evolved. And yet that data is still insufficient to determine an accurate absolute geological age for the migration.
Our innovation is based on uranium-lead (U-Pb) radiometric dating, a method that is especially suited to carbonate rock but until now could not be used for reservoir rock. That’s because the crystals in the relevant cements are often too small (between 100 and 500 microns) and do not contain large enough amounts of uranium-lead. But today, using in situ laser ablation, we can perform U-Pb measurements directly on a thin section of rock, with a resolution of 200 microns.
These thin section images show a diagenetic dolomite crystal containing abundant liquid oil inclusions (see arrows).
In such a situation, by measuring the absolute age of the dolomite, we can determine when the oil accumulated.
Applying the U-Pb Methodology to Pre-Salt Reservoirs: Success in Angola, Prospects in the South Atlantic
The value of this methodology has been borne out by research conducted on an offshore exploration well drilled to a depth of more than 5 kilometers in Angola. Those studies show that U-Pb geochronological dating represents a true paradigm shift, one that completely changes both our understanding of thermal history and our model of the Angolan basin.
- Prior to drilling, there was no evidence of any abnormal thermal events in the pre-salt reservoirs that might have resulted from the local volcanism. As a result, the initial basin models did not anticipate any maturation of the pre-salt source rock in the immediate vicinity of the prospect before Oligocene time.
- After drilling, thanks to the use of the U-Pb methodology on several cements from the Chela-Cuvo formations, it became possible — for the first time ever — to arrive at an absolute date for the diagenetic dolomite and silica in a petroleum reservoir.
Using those ages, we were able to establish the existence of a major thermal peak during the Albian and Cenomanian times (112-92 Ma) and date the emplacement of hydrocarbons to 99 Ma; in other words, the oil accumulated at a very early stage.
Therefore, we now know that a significant column of oil was already present in the pre-salt reservoirs about 15 million years after they were deposited. That scenario, which had not been considered previously, changed our view of the petroleum system’s origin and evolution, and the area’s prospectivity.
Our new methodology thus offers critical added value for our exploration strategy in the South Atlantic pre-salt reservoirs. Potentially applicable to every petroleum system worldwide, it will yield significant cost savings over time and improve our success rate across all of our acreage.
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