Thermal reconstruction - Applications and implications for O&G exploration
The Andean Amazon Basin (AAB) is the perfect example to illustrate how to investigate oil & gas potentials. The zone of study was the northern Sub-Andean Zone of Ecuador or western uplifted part of the AAB (Napo uplift; Fig. 4). In the AAB, source rocks are Palaeozoic and Middle Cretaceous black shales whereas reservoir rocks vary from early Cretaceous to Eocene sandstones.
Figure 4: 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. 5).
Cross-section to the bottom left was extracted from the southern Sub-Andean Zone till the Andean Amazon Basin (dashed B&W line; courtesy of IRD and PetroEcuador).
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 4 & 5)
Geochemistry of the mineral analysed
Reverse thermal modeling is the end product. It is completed using a free software from the Uni. of Texas.
As illustrated in figures 2 and 5, 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 (Fig. 5).
Temperature-time paths for the Jurassic substratum of the reservoir Hollin Fm. and hence of the Hollin Fm. since deposition.
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 reservoir did not stay long enough in the oil temperature window (160-60°C). Hence oil most likely migrated from below or laterally. The other third yielded a clear overburden due to the growing Eastern Cordillera with temperatures higher than of 200°C in the Cretaceous-Eocene.
The thermal reconstruction of oil dripping outcrops in the Sub-Andean Zone all show within the oil temperature window (160-60°C):
A phase of cooling (eruption or exhumation?) 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.
An ultimate phase of cooling/exhumation in the Oligocene-early Miocene.
Oil migration occurred during the ultimate phase of cooling/deformation in the Oligocene-Miocene .
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 any rock that hosts carbonate material such as black shales and carbonates in an O&G rich basin (Fig. 6).
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).
Since 2006 we systematically combine Raman Spectroscopy and low-Temperature thermochronological analyses in all projects such as in North Africa, Zagros and Caucasus in basins bordering the Sahara and Atlas systems. We produced a huge database from western Morocco (Figure 7) that allowed us to constrain in details the thermal histroy of the Atlas-basin(s) system.
Raman Spectroscopy permits to the obtained the maximum peak temperature for a shale or a schist. Once this temperature known, we date when this maximum temperature was reached using the accurate thermochronometer. Example: if Peak temperature is ~180°C, which is in the middle of the gas temperature window (200-160°C) we complete U-Th-Sm/He analyses on zircon (Fig. 7) to get this age because its metastable zone is between 200-160°C.
Additional thermochronological analyses are required from the same morpho-structural domain to obtain its paleo-thermal history from 200 till 45°C. If the maximum temperature is 230°C, hence this is overcooked with all its kerosene potential lost.
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
- We cannot go into details of the thermal reconstruction of gas rich levels because of intellectual property but we are opened to discussion and would present it on an individual basis. The approach is very similar to the one for the Thermal reconstruction within the Oil window (see above).
- Specific targeted lithologies for thermal reconstructions are siltstones, sandstones, conglomerates but also volcano-sedimentary rocks, plutons, metamorphics (highest apatite and zircon yield). Thermal paths are classically reconstructed between the 160-55°C temperature window, which is roughly the oil temperature window (OTW, Fig. 4 & 5). To investigate higher temperatures we elaborated a novel approach that allows us to reconstruct from the same location, e.g. well, outcrop, or from the same anticline thermal paths for higher temperatures until 200°C temperature and even till 310°C in certain circumstances.
The thermal reconstruction of any shale or schist series requires more analyses than O&G rich sediments. We can however determine for all 1) phases of burial, 2) the maximum temperature reached, 3) exhumation but also 4) the residency of source and reservoir rocks within the oil and gas temperature windows.
• 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. It can be applied to onshore and offshore basins and thrust and fold-belt.
Furthermore, we can also determine rate of any geological processes occurring in any orogen-basin system. It necessitates a perfect comprehension of the 1) geology of the region, 2) methods and their limits. GeoLogin 3G developed this expertise.
• The temperature windows for oil and gas are 160-60°C and 200-160°C respectively but they vary with pressure and the composition of organic material. Our expertise allows the thermal history of O&G rich basins to be reconstructed from 200°C to 45°C so within the Oil and Gas temperature windows. Hence, we can propose you to estimate the potential of conventionnal and unconventional reservoirs thanks to the combination of a geothermometer and low-thermochronology analyses.
• Our 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 as cores or cuttings extending possible analyses to any well so to many more past and present O&G projects.
• We aimed this booklet to any geoscientist from O&G industry. As geologists, we would feel fulfilled with all these novel analyses, possibilities, applications and perspectives.