Low resistivity zones at contacts of igneous intrusions emplaced in organic‐rich formations and their implications on fluid flow and petroleum systems: A case study in the northern Neuquén Basin, Argentina

Abstract

Igneous sills and laccoliths emplaced in sedimentary basins may significantly impact petroleum systems, both positively and negatively. Igneous intrusions provide heat to maturate regionally immature organic-rich host rocks, act as fractured reservoirs hosting commercial accumulations of hydrocarbons, and form structures affecting fluid flow and trapping at different scales. Nevertheless, the petrophysical implications of igneous intrusions on their host rock are poorly known. In this study, we analyse 200 wells in the Río Grande Valley oil field, Neuquén basin, Argentina, where the main reservoirs are in fractured igneous sills. This dataset represents a globally unique possibility to characterize the igneous-host rock interaction using both wireline logs and core material. We identify a systematic Contact Low Resistivity Zone (CLRsZ) at both the upper and lower contacts of the sills emplaced in the organicrich Vaca Muerta and Agrio Formations. We characterize the nature of these CLRsZ and their petrophysical properties by integrating resistivity and gamma ray well logs, petrographic analyses, petrophysical tests and geochemical analyses. The low resistivity signal of the CLRsZ is dominantly carried by massive-sulphide deposits, mainly pyrite, observed both in the host rock and the chilled margin of the sills. Well log images and porosity-permeability analysis on core plugs show that both the sills and their associated CLRsZ can act as carrier for fluid flow and reservoir for hydrocarbons storage. The thickness of the upper and lower CLRsZ correlates linearly with the thickness of the sill, and the volume of both the upper and lower CLRsZ represents ca. 40% with respect to the volume of their associated sill. The thickness of the CLRsZ represents ca. 13% of the thickness of contact aureole induced by the sills. In the CLRsZ, a great proportion of kerogen was transformed to hydrocarbon, so that CLRsZ were restricted to the innermost contact aureole of the sills. Our results show that the CLRsZ can have major implications on fluid flow and should be considered in reservoir models in volcanic basins hosting sills emplaced in organic-rich formations. 4 | EAGE SPACAPAN et Al.

Stratigraphic and Spatial Extent of HALIP Magmatism in Central Spitsbergen

“…The thick sill is clearly discernible on the wireline data, with low gamma ray and high resistivity and velocity. Notably, both contact aureoles are characterized by very low resistivity, which may be related to inter-connected pyrite or graphite as described recently from the Neuquén Basin by Spacapan et al (2019). A similar signature, though only in the lower aureole, is apparent in the Colesbukta borehole (Figure 15).…”

“…Numbers list scientific questions identified in this manuscript and indicate the locations in the HALIP plumbing system to address them. Miranda et al, 2018;Spacapan et al, 2018Spacapan et al, , 2019. Assessing the nature of the environmental effects of large igneous provinces thus requires a solid knowledge of the lithology of the host rock of the LIPs plumbing system, the documentation of fluid release structures and good control on the composition and emplacement history of the magma.…”

Impact of permeability evolution in igneous sills on hydrothermal flow and hydrocarbon transport in volcanic sedimentary basins

“…First, mineral precipitation at high temperatures can occur at nonnegligible rates and lead to fast porosity-decrease, causing pore pressure increase and possibly fracturing (Townsend, 2018). Fracturing in the aureole may not be reversible but permanently increase porosity and permeability, which has been documented in systematic core studies of contact aureoles in the RGV (Spacapan et al, 2019). These features should be included in future research, and the model predictions tested against field and laboratory studies.…”

Numerical Modeling of Fracture Network Evolution in Organic‐Rich Shale With Rapid Internal Fluid Generation

“…For each case of the initial fracture geometry, we ran simulations with increasingly anisotropic stress boundary conditions, representing a constant depth of around 2.5 km. This approximately corresponds to the onset of the oil window as well as an intermediate emplacement depth for intrusions that could cause fast maturation of oil and gas (Spacapan et al, 2018(Spacapan et al, , 2019. Starting from applying isotropic stress of 67.5 MPa along all boundaries, we decrease the horizontal stress by 1%, 5%, 10%, and 200%.…”

“…Fractures representing fluid pathways in tight, organic-rich shales are well documented from both field and borehole data in both standard burial and volcanic settings (Marquez & Mountjoy, 1996;Ougier-Simonin et al, 2016;Spacapan et al, 2019;Zanella et al, 2015). However, our current understanding of hydrofracturing due to internal fluid generation stems primarily from laboratory models and a few numerical studies.…”