DEFORMATION BANDS AND THEIR INFLUENCE ON FLUID FLOW IN TIGHT-GAS SANDSTONE FROM LAJAS FORMATION IN THE HUINCUL HIGH, NEUQUÉN BASIN

RAINOLDI ANA; NATALIA FORTUNATTI; DANIEL MINISINI; MARTA FRANCHINI

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

UNPSJB, CONICET-MEF, UNRN, YPF

The Neuquén Basin is the main hydrocarbon producing basin of Argentina. The basin is subdivided in diff erent morphostructural units according to their distinctive evolutionary characteristics: Andean foldand-thrust belt, Los Chihuidos High, Neuquén Embayment, Huincul High and Outer Shelf. The Huincul High represents a deformation belt that consists of half-graben faults formed in a right-lateral transpressive system (Mosquera and Ramos 2006). This deformation belt hosts several oil and gas fi elds, among them the tight-gas sandstones of Lajas Formation, where the presence of deformation bands could have had a great impact on fl uid migration and accumulation.Deformation bands are meso-scale tectonic/diagenetic structures that frequently accompany major faults in porous sedimentary rocks (Exner et al. 2013 and references therein). According to their kinematic properties, deformation bands can be classifi ed as shear, compaction and dilation bands, with hybrids between these end members (Aydin et al. 2006). Pore reduction is generally expected in deformation bands, however if a component of dilation is involved, an increase of porosity and permeability could occur. This study examines deformation bands in cores of the Lajas Formation located in the Sierra Barrosa oil fi eld (Huincul High) to identify their infl uence on hydrocarbon migration and reservoir quality. In the study area, sandstones of the Lajas Formation consist of moderately-sorted litharenite with clayey matrix, cemented by quartz, sulfi des, feldspar and carbonates. The latter show common impregnations of bitumen and hydrocarbon-bearing fl uid inclusions. The rocks are strongly compacted and display low porosity as evidenced by pressure-solution contacts and microstylolitization surfaces. Dilation and compaction bands are identifi ed in the analyzed cores.Dilation bands involve a shear component, therefore are classifi ed as shear-enhanced dilation bands (SEDB). According to the degree of cementation, two types of SEDB are recognized: cemented (SEDB-C) and not cemented (SEDB-nC). Both types have high angle respect to bedding. Cemented shear-enhanced dilation bands (SEDB-C) are characterized by low porosity due to grain crushing and siderite precipitation. Siderite fi lled the pores and replaced detrital grains. The degree of cementation varies along the structure, from weakly cemented to zones characterized by relict grains completely embedded in siderite. Siderite occurs as well-developed prismatic crystals with non-luminescent and red luminescence zones under ultraviolet light. The remaining space is fi lled with kaolinite. Both siderite and kaolinite are impregnated with bitumen. Not-cemented shear-enhanced dilation bands (SEDB-C) present dark color in core due to bitumen impregnations. Microscopic studies show a discontinuous pattern of the structure characterized by aligned crushed quartz grains impregnated with bitumen that alternate with unbroken lithic and feldspar grains. Minor siderite and kaolinite impregnated with bitumen could be observed.Hydrocarbon-bearing fl uid inclusions were captured in dilational bands. In the SEDB-C, primary fl uid inclusions occur in siderite and secondary fl uid inclusions are hosted in relict detrital lithic and feldspar grains. In the SEDB-nC only secondary aligned fl uid inclusions were recorded in detrital quartz grains. In all cases hydrocarbon-bearing fl uid inclusions are colorless, display two phase (L+V) and emit light-blue fl uorescence under ultraviolet light.Compaction bands could lack shear component (named pure compaction bands, PCB), present lowto-moderate intensity of shear deformation (named shear-enhanced compaction bands, SECB), or high intensity of shear deformation (named compactional shear bands, CSB). Pure compaction bands (PCB) have high-angle respect to bedding and show signifi cant less porosity than the host sandstone. The absence of quartz, feldspar, sulfi de and carbonate cements in the deformation band compared with the host rocks, indicates that PCB formed before cementation. Shear-enhanced compaction bands (SECB) and compactional shear bands (CSB) are parallel or have low-angle respect to bedding. These deformation bands underwent signifi cant loss of porosity due to grain-size reduction and clay formation related to cataclasis processes; since the degree of cataclasis is related to the degree of shear, SECB presents low degree and CSB presents high degree of cataclasis. Cataclastic deformation bands are cut and displaced by mode II/III fractures, with carbonate cemented slickensides.