Relating Acoustic Anisotropy to Kerogen Content in Unconventional Formations - A Case Study in A Kerogen-Rich Unconventional Carbonate

The mechanical and petrophysical behavior of organic-rich carbonates (ORC) is affected significantly by burial diagenesis and the thermal maturation of their organic matter. Therefore, establishing Rock Physics (RP) relations and appropriate models can be valuable in delineating the spatial distribution of key rock properties such as the total organic carbon (TOC), porosity, water saturation, and thermal maturity in the petroleum system. These key rock properties are of most importance to evaluate during hydrocarbon exploration and production operations when establishing a detailed subsurface model is critical. High-resolution reservoir models are typically based on the inversion of seismic data to calculate the seismic layer properties such as P- and S-wave impedances (or velocities), density, Poisson's ratio, Vp/Vs ratio, etc. If velocity anisotropy data are also available, then another layer of data can be used as input for the subsurface model leading to a better understanding of the geological section. The challenge is to establish reliable geostatistical relations between these seismic layer measurements and petrophysical/geomechanical properties using well logs and laboratory measurements. In this study, we developed RP models to predict the organic richness (TOC of 1-15 wt%), porosity (7-35 %), water saturation, and thermal maturity (Tmax of 420-435⁰C) of the organic-rich carbonate sections using well logs and laboratory core measurements derived from the Ness 5 well drilled in the Golan Basin (950-1350 m). The RP models are based primarily on the modified lower Hashin-Shtrikman bounds (MLHS) and Gassmann's fluid substitution equations. These organic-rich carbonate sections are unique in their relatively low burial diagenetic stage characterized by a wide range of porosity which decreases with depth, and thermal maturation which increases with depth (from immature up to the oil window). As confirmation of the method, the levels of organic content and maturity were confirmed using Rock-Eval pyrolysis data. Following the RP analysis, horizontal (HTI) and vertical (VTI) S-wave velocity anisotropy were analyzed using cross-dipole shear well logs (based on Stoneley waves response). It was found that anisotropy, in addition to the RP analysis, can assist in delineating the organic-rich sections, microfractures, and changes in gas saturation due to thermal maturation. Specifically, increasing thermal maturation enhances VTI and azimuthal HTI S-wave velocity anisotropies, in the ductile and brittle sections, respectively. The observed relationships are quite robust based on the high-quality laboratory and log data. However, our conclusions may be limited to the early stages of maturation and burial diagenesis, as at higher maturation and diagenesis the changes in physical properties can vary significantly.

Diamondoids and Thiadiamondoids Generated From Hydrothermal Pyrolysis of Crude Oil and TSR Experiments

Diamondoid compounds are widely used to reflect thermal maturation of high mature source rocks or oils and oil cracking extents. However, diamondoids and thiadiamondoids were demonstrated to have newly been generated and decomposed in our hydrothermal pyrolysis of crude oil and TSR experiments. Our results show that adamantanes and diamantanes are generated primarily within the maturity range 0.48–2.1% and 1.2–3.0% EasyRo, respectively. Their formation is enhanced and the decomposition of diamantanes obviously lags behind at elevated temperatures compared with anhydrous experiments. MDI, EAI, DMAI-1, DMDI-2 may serve as reliable maturity proxies at > ca.1.0% EasyRo, and other isomerization indices (TMAI-1, TMAI-2 and DMAI-2) are effective for the highly mature organic matter at EasyRo > 2.0%. The extent of oil cracking (EOC) calculated from the broadly used 3-+4-MD method (Dahl et al., 1999) is proven to overestimate, especially for highly cracked samples due to the new generation of 3-+4-MD. Still, it can be corrected using a new formula at <3.0% EasyRo. Other diamondoid-related indices (e.g. EAI, DMDI-2, As/Ds, MAs/MDs, DMAs/DMDs, and DMAs/MDs) can also be used to estimate EOC. However, these indices cannot be applied to TSR-altered petroleum. TSR is experimentally confirmed to generate diamantanes and thiaadmantanes at 1.81% EasyRo via direct reactions of reduced S species with hydrocarbons and accelerate the decomposition of diamantanes at > 3.0% EasyRo compared with thermal chemical alteration (TCA).

Geochemical Considerations from the Carboniferous Unconventional Petroleum System of SW Iberia

The Baixo Alentejo Flysch Group (BAFG) is an important stratigraphic unit that covers over half of the South Portuguese Zone (SPZ) depositional area, and it is composed by three main tectono-stratigraphic units: the Mértola, Mira, and Brejeira formations. All of these formations contain significant thicknesses of black shales and have several wide areas with 0.81 wt.%, 0.91 wt.%, and 0.72 wt.% average total organic carbon (TOC) (respectively) and thermal maturation values within gas zones (overmature). This paper is considering new data from classical methods of organic geochemistry characterization, such as TOC, Rock–Eval pyrolysis, and organic petrography, to evaluate the unconventional petroleum system from the SPZ. A total of 53 samples were collected. From the stratigraphical point of view, TOC values seem to have a random distribution. The Rock–Eval parameters point out high thermal maturation compatible with gas window (overmature zone). The samples are dominated by gas-prone extremely hydrogen-depleted type III/IV kerogen, which no longer has the potential to generate and expel hydrocarbons. The petrographic analyses positioned the thermal evolution of these samples into the end of catagenesis to metagenesis (wet to dry gas zone), with values predominantly higher than 2 %Ro (dry gas zone). The presence of thermogenic hydrocarbon fluids characterized by previous papers indicate that the BAFG from SPZ represents a senile unconventional petroleum system, working nowadays basically as a gas reservoir.

Análisis geoquímico y petrográfico de las formaciones Los Cuervos y Molino, Subcuenca Cesar (Colombia): implicaciones en la evolución del sistema petrolero

A geochemical characterization of Los Cuervos and Molino formations in the Cesar Sub-Basin was carried out using core samples obtained from the ANH-La Loma-1 Well. A total of 113 Rock-Eval pyrolysis analysis, total organic carbon (TOC), total sulphur content (TS) analysis, 13 vitrinite reflectance analysis (%Ro) and 30 thin-section petrographic analysis were performed. Based on these new data, it was possible to classify the quality of organic matter and the current thermal maturation of Los Cuervos and Molino formations. Additionally, a petrographic characterization of 30 samples allowed the correlation of the lithology with the geochemical results. Also, one-dimensional geochemical modelling was implemented in order to contribute to the knowledge of the evolution of the oil system in the Cesar Sub-Basin. The spatial distribution of the formations used in the modelling was obtained from 2 seismic lines two-way time. The results obtained indicate that Los Cuervos Formation presents TOC values from 0.29 to 66.55%, TS values from 0.02 to 11.29%, their organic matter consisted of type III kerogen which is consistent with an immature thermal maturation stage. In contrast, the Molino Formation presents TOC values from 0.23 to 2.28%, TS values from 0.001 to 1.39%, their organic matter consisted of type II/III kerogen this suggests an early entry to the oil window with a maximum pyrolysis temperature (Tmax) value of 442°C. The geochemical modelling tunes better with measured data from palaeo-geothermometers (%Ro and Tmax). The geochemical modelling shows that, between 60 - 40 million years ago (mya), the Cretaceous formations entered in the oil generation window and it is expected that, between 40 - 30 mya, the Lagunitas, Aguas Blancas, and La Luna formations will be at their peak of hydrocarbon generation.