Thermal history | GeoLogin 3g

Thermal reconstruction - Applications and implications for O&G exploration

Example from Ecuador

The zone of study is the northern Sub-Andean Zone of Ecuador or western uplifted part of the Andean Amazon Basin (AAB; Fig. 2). In the AAB, source rocks are Palaeozoic and Middle Cretaceous black shales whereas reservoir rocks vary from early Cretaceous to Eocene sandstones.

Figure 2: the Ecuadorian Andean Amazon Basin with its proximal (Sub-Andean Zone) and distal parts (Amazon Basin).

The location of productive and non-productive fields are courtesy of PetroEcuador.

Left: details of the 60 My hiatus between the Jurassic volcanic arc and the Cretaceous basin fill series in the Sub-Andean Zone. Red rectangle: localization of the outcrop to the left from which thermal reconstruction was completed (Fig. 3).

Cross-section to the bottom left from Baby et al. (2013).

Input parameters for thermal reconstruction are:

Thermochronological analyses: Fission-Track + U-Th-Sm/He analyses on Apatite and zircon.
Geological constraints (e.g. hiatus, an unconformity...; Figures 2 & 3)
Geochemistry of the mineral analysed

Reverse thermal modeling is the end product. It is completed using free softwares from the Uni. of Texas or Rennes (Ketcham, 2005 & 2024; Gallagher, 2012)

As illustrated in figure 2 (Tar sands), the oil-dripping reservoir of Hollin Fm. is in unconformable contact with the Jurassic substratum of the Andean Amazon Basin. The Hollin Fm. is made of beach sandstones deposits that do not host any apatite because they are too mature, i.e. apatites were destroyed during transport. Such scenario usually prevented any interpretation. However we found a way to reconstruct the thermal history of this reservoir by analysing apatites and zircons from the substratum.

Figure 3:

Temperature-time paths for the Jurassic substratum of the reservoir Hollin Fm. and hence of the Hollin Fm. since deposition. Black rectangle geological constraints.

1, 2, 3, 4 are the four metastable zones associated to the 4 low-temperature thermochronometers .

AFT & ZFT: Apatite and Zircon Fission-Track analyses. AHe, ZHe; U-Th-Sm/He analyses on apatite and zircon respectively.

Right: outcrop picture for the Jurassic-Cretaceous contact with the basal conglomerates of Aptian age.

Such approach was repeated on any substratum-Cretaceous outcrops we encountered to produce a large overview of the thermal evolution of the Andean Amazon Basin. Two thirds of the thermal models indicate that the source rocks stay long enough in the oil temperature window (160-60°C).

Conclusions

The thermal reconstruction of tar sands in the Sub-Andean Zone all show:

A phase of cooling (eruption of the Jurassic volcanics) that began in the early Jurassic and ended in the Late Jurassic
A phase of heating associated to sedimentary burial from Aptian-Albian until the Late Eocene into the oil window.
An ultimate phase of exhumation in the Oligocene-early Miocene that corresponds most likely to oil migration

Determination of Peak Temperature, its age and implication for shale gas exploration

Researchers from the University of Paris VI and ENS developed in 2002 a new geothermometer: Raman Spectroscopy analyses on Carbonaceous Material. The principle is simple: carbonaceous material is getting organised with temperature increasing. This process is irreversible and permits to determine the maximum temperature reached by rock that hosts carbonate material such as black shales and carbonates (Fig. 4).

This geothermometer is valid for a large temperature window that ranges from 600°C to 150°C, the latter value being almost the upper bound of the oil temperature window (160-60°C) and lower bound of the gas one (200-160°C).

Raman Spectroscopy permits to obtain the maximum peak temperature for a shale or a schist. Once this temperature known, I can date when this maximum temperature was reached using the accurate thermochronometer. Example: if Peak temperature is ~180°C, I can complete U-Th-Sm/He analyses on zircon to get this age because its metastable zone is between 200-160°C.

If the maximum temperature is 230°C - it is overcooked and all its kerosene potential is lost.

Figure 4.

Schematic sketch of natural gas resources. A: gas associated to oil reservoirs; B: conventional gas; C: coal gas; D: gas encountered in ultra-compacted reservoir; E: shale gas

Thermal reconstruction of gas rich layer

The approach is very similar to the one for the Thermal reconstruction within the Oil window but requires analyses on samples above or below the layer hosting the gaz.

Conclusions

- Specific targeted lithologies for thermal reconstructions are siltstones, sandstones, conglomerates but also volcano-sedimentary rocks, plutons, metamorphics (highest apatite and zircon yield).

- The combination of low-temperature thermochronology and Raman Spectroscopy methods is of major interest for O&G industry because it fills a void in the thermal aspect of O&G basins rich in carbonate or shales. It can be applied to onshore and offshore basins and thrust and fold-belt.

- Furthermore, I can also determine rate of geological processes, e.g. exhumation or burial occurring in any orogen-basin system. It necessitates a good comprehension of the 1) geology of the region and 2) methods and their limits.

- My expertise allows the thermal history of O&G rich basins to be reconstructed from 310°C to 55°C so within the Oil and Gas temperature windows.

- My approach is being integrated into the line of command of some O&G companies. This approach generates shortcuts, avoid unnecessary investigations and save time and money. In addition, technology brings us some help with a new machine that fragments samples at the grain boundaries and preserves the shape of all minerals. Such machine allows the separation of much smaller samples with a higher yield in apatite and zircon. We can now decrease the weight of a sample to 1.5-2 kgs like cores or cuttings extending possible analyses to any well so to many more past and present O&G projects.