Thermal effect by igneous intrusions on the organic matter of the Vaca Muerta Formation, Neuquén Basin, Argentina

PINEDA JUAN; SPACAPAN JUAN; COMERIO MARCOS; FRAYSSINET LUCRECIA; LAPCAK BARBARA

Congreso; 38th Annual Meeting of The Society for Organic Petrology; 2022

The Vaca Muerta Formation (Late Jurassic–Early Cretaceous) constitutes the most prolificsource rock of the Neuquén Basin, Argentina. It is considered a world-wide unconventional shaleplay due to its high-quality, oil-prone kerogen, deposited under mostly anoxic, marine condition.Recent studies have shown that organic matter plays a fundamental role in organic porositydevelopment and thus exert a main control on oil and gas storage capacity. The evolution oforganic porosity during thermal maturation by burial diagenesis is evidenced from the early oilwindow to the dry gas in the basin, however, this evolution in areas affected by igneousintrusions has not been addressed. The present study focuses on the Vaca Muerta Formationbased on subsurface data, where horizontal igneous intrusions (sills) enhanced source rockmaturation and act as the main oil reservoirs in the Mendoza area, Argentina, of the basin. Theobjective is to evaluate changes on the kerogen composition across the thermal effects inducedby intrusions and the development of organic pores related to specific macerals. Core samplesfrom nearby Well A and Well B (affected by sill intrusions) were analyzed for maceral analysis,vitrinite reflectance (%Ro), Rock-Eval, scanning electron microscope (SEM), and combined withbasin and petroleum system modelling (BPSM). Well A shows average values of TOC (3.11 wt%), S2 (14.09 mgHC/rock), HI (441.17 mgHC/gTOC), and is in the early oil window (avg. Tmax 438℃ and 0.7 %Ro). Contrary, the zone disturbed by the intrusive in Well B shows anomalies in geochemical parameters such as: TOC (avg. 1.96 wt%), S2 (avg. 2.40 mgHC/rock), HI (avg. 125.75 mgHC/gTOC), and Tmax (avg. 417 °C). These parameters deviate from the expected values for this region of the basin, which causes the oil window to shift to a post-mature stage (%Ro >3.4, close to the intrusive). Well A is characterized by kerogen type I/II, dominated by liptinite macerals including discrete alginite bodies, lamellar and filamentous lamalginite, liptodetrinite and bituminite. solid bitumen (SB) is present and occurs as void or fracture filling. In the non-perturbed zone by intrusions, Well B has a similar kerogen composition to well A. Kerogen start to change from liptinite to abundant groundmass SB towards the contact with the sills, which is evidenced by a strong decrease of HI value and the presence of geochemical anomalies data. Evolution of organicporosity is observed under SEM and organic pores develop close the intrusion. Identifying therelation between organic petrography, Rock-Eval and SEM combined with BPSM provides anexploration model that can be useful to evaluate potential oil and gas shale reservoirs associatedwith igneous intrusions.