New integrated approach to assess thermal maturity of sedimentary succcessions by means of organic and inorganic indicators

Abstract

The assessment of thermal maturity in sedimentary successions is crucial for a correct calibration of thermal modelling. Uncertainties in thermal maturity can thus strongly affect the reliability of geological studies on sedimentary basins or influence negatively decisions in hydrocarbon (HC) exploration. Pitfalls in vitrinite reflectance, the most used thermal parameter used to calibrate thermal history, are one of the main cause of such uncertainties. In this work we propose a multimethods approach in order to overcome problems related to the absence, scarceness of vitrinite macerals in an organic facies or to the scarce reliability of vitrinite reflectance data. The approach is based on thermal maturity indicators carried out from the analyses on both the organic (e.g. Pyrolysis Rock Eval, Fourier Transform Infrared Spectroscopy and Raman spectroscopy) and the inorganic (e.g. clay mineralogy, low-temperature thermochronology) fraction of sediments. The classical thermal maturity indicators have been correlated among them and then used to calibrate 1D thermal models in order to unravel the thermal history of studied successions and model the generation/expulsion of HC from potential source rocks. Most of the methodologies adopted in this work are well known and a vast literature exists on their use, although many limitation can affect them. Therefore totally novel parameters derived from Raman spetroscopic analyses on the organic fraction of sediments have been tested and correlated with different levels of hydrocarbon generation, in the immature and mature stages. In order to calibrate new Raman parameters against more robust ones in a wide range of thermal maturity and on different kinds of organic matter (OM), four areas have been studied. They are characterised by Paleozoic, Mesozoic and Cenozoic successions that have been sampled both in surface and subsurface. In detail in the first case history, 33 cuttings from Oligocene to Upper Miocene successions from a well drilled in the Lower Congo Basin, in the offshore Angola, were analysed. The paucity of vitrinite-rich samples along the well and the occurrence of vitrinite reflectance retardation phenomena, due to the development of overpressure during the deposition of the Congo fan-delta complex and/or to the presence of high hydrogen content in the OM, make the measured vitrinite reflectance data scarcely useful for modelling. In order to overcome this problem we modelled thermal maturity across the well using the kinetic equation proposed by Cuadro and Linares (1996) based on measured values of Illite % in the Illite-smectite mixed layers (%I in I-S). The best fit was acquired with a thermal evolution developed according to a single geothermal gradient similar to the present-day one. 2 To verify the maturation of the organic matter we also applied Fourier Transform Infrared Spectroscopy (FT-IR), that is a useful tool when dealing with amorphous organic matter-rich (AOM) kerogen. Results indicate a regular increase of both aromaticity and aliphaticity parameters confirming thus the evolution carried out from mineralogical indicators and their modelling. The second case history comprises a sector of the Carpathians fold-and-thrust belt. Here 19 samples were collected with ages comprised between Lower Cretaceous and Miocene. The organic matter dispersed in sediments shows a very complex composition due to the presence of several macerals groups . This makes difficult to discriminate vitrinite from other similar macerals during organic petrography analyses. In this case Tmax from Pyrolysis Rock Eval analyses was coupled with optical investigations to define thermal maturity. Ro% and Tmax data were used to calibrate the maturation patterns in thermal models performed for two tectonic units cropping out in the study area (Chornogora and Skiba units). Thermal modelling and Pyrolysis Rock Eval results pointed out that several oil and gas prone source rocks are developed in the area and that their maturity falls in the immature or early mature stage of HC generation. Furthermore , the burial history of these two tectonic units suggests that, in the most internal Chornogora unit, the outcropping rocks preserve their pre-overthrusting level of thermal maturity, while in the external Skiba unit an additional loading of about 1,500m must have occurred during alpine deformation. This load of probable tectonic origin emplaced at the beginning of the overthrusting phase, since Late Miocene times. Paleozoic samples were collected in successions comprised in age from Cambrian to Devonian that crop out in the Holy Cross Mountains (central Poland). This area is characterised by the outcrop of two tectonic units, called Kielce block to the south and Łysogory blocks to the north. They are separated by a major fault, called the Holy Cross Fault (HCF). Due to the absence of upper plants until Devonian times, Lower Paleozoic successions in the study area are devoid of vitrinite. Nevertheless, it has been possible to performed optical analyses on marine organoclasts other than vitrinite, as graptolites. According to the parametrization proposed by several authors (Bertrand, 1990; Bertrand and Malo, 2012; Bustin et al., 1989; Goodarzi and Norford, 1987; Suchý et al., 2002) their reflectance values have been transformed in vitrinite relectance equivalent data. These data together with Tmax, clay mineralogy and Raman parameters, pointed out a significant difference in thermal maturity between the two blocks, in agreement with previous authors (Belka, 1990; Dadlez, 2001; Marynowski et al., 2001; Narkiewicz, 2002; Szczepanik, 1997, 2001). Thermal models in the two blocks indicate that they both suffered two main phases of burial: one at the end of Paleozoic and the other at the end of Mesozoic times. The two models, performed to evaluate the potential of Lower Silurian shales as gas bearing source rocks, evidenced that in the Łysogory region maturity necessary to generate gas was reached during the first burial event, while in Kielce block it was reached more recently during Mesocoiz times. In addition, Pyrolysis 3 Rock Eval data indicate that only Silurian , Devonian rocks in the Kielce region could act as fair to good source rocks. 18 Ordovician to Devonian samples were collected in the Podolia region in Ukraine. This area offers a good example on how we can obtain thermal maturity indicators from mineralogical analyses, useful to calibrate a thermal models, when organic matter dispeserd in sediments is very scarce and source rocks do not occur. % I in I-S data were used as thermal maturity indicators of maximum burial, while low T thermocronological data were used to costrain the time of the exhumation events. Three models were performed showing a progressively deepening of the basin (eroded thickness from 3000 to 3700) moving from east to the west. Exhumation occured in Late Triassic-Early Jurassic times. Such an exhumation event cannot be relataed to either the Variscan or the Alpine main phases of deformation whose field evidences crop out close to the Podolia region. On the other hand we argue that the main cause of exhumation in the region is related to an important far-field stress derived from the Cimmerian ororgenesis whose collisional front is located some hundreds of km away. Once correctly defined thermal maturity of samples for the different case histories, it was possible to correlate them with parameters carried out from Raman spectroscopy. Our results demonstrate for the first time that Raman spectra of undifferentiated dispersed organic matter (excluded macerals of the inertinite group) show quantifiable changes in response to thermal maturation and can be successfully used to parameterize thermal evolution, even at very low diagenetic stages, between immature and mid mature stages of hydrocarbon generation. In particular two successful parameterizations against vitrinite reflectance have been determined based on the area or width ratio of the bands that compose Raman spectra. We argue that variations in the Raman spectra of kerogen in diagenesis are related to completely different mechanisms with respect to those that occurs during carbonization in metamorphism and graphitization.