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

RAINOLDI, ANA LAURA; FORTUNATTI, NATALIA; MINISINI, DANIEL; FRANCHINI, MARTA

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

The Neuquén Basin is the main hydrocarbon producing basin of Argentina. The basin is subdivided in different morphostructural units according to their distinctive evolutionary characteristics: Andean fold-and-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 fields, among them the tight-gas sandstones of Lajas Formation, where the presence of deformation bands could have had a great impact on fluid 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 classified 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 field (Huincul High) to identify their influence 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, sulfides, feldspar and carbonates. The latter show common impregnations of bitumen and hydrocarbon-bearing fluid inclusions. The rocks are strongly compacted and display low porosity as evidenced by pressure-solution contacts and microstylolitization surfaces. Dilation and compaction bands are identified in the analyzed cores. Dilation bands involve a shear component, therefore are classified 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 filled 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 filled 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 fluid inclusions were captured in dilational bands. In the SEDB-C, primary fluid inclusions occur in siderite and secondary fluid inclusions are hosted in relict detrital lithic and feldspar grains. In the SEDB-nC only secondary aligned fluid inclusions were recorded in detrital quartz grains. In all cases hydrocarbon-bearing fluid inclusions are colorless, display two phase (L+V) and emit light-blue fluorescence under ultraviolet light.Compaction bands could lack shear component (named pure compaction bands, PCB), present low-to-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 significant less porosity than the host sandstone. The absence of quartz, feldspar, sulfide 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 significant 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.By definition, deformation bands affect highly-porous sediments and poorly-lithified rocks (Fossen et al., 2007). In the case of the Lajas reservoirs, the absence of earlier host rock cements (i.e. quartz, feldspar, sulfides), in both compaction and dilation bands, indicates that deformation bands formed during early diagenesis when the unit was still highly porous. In the case of dilation bands, porosity and permeability increased, whereas in compaction bands they decreased. With increasing burial, hydrocarbons migrated through the dilational deformation bands, as supported by the hydrocarbon-bearing fluid inclusions and the abundant bitumen impregnations in their siderite and crushed quartz grains. During this stage, organic acids and CO2 associated with hydrocarbons could have promoted grain dissolution, enhancing the porosity of the dilational bands, and precipitation of siderite cement. Instead, the poorly-porous compaction bands acted as barrier for fluid flow that migrated through the host rocks, as documented by their bitumen impregnations and carbonate cement. According to Mosquera et al. (2011), the main stress field orientation during the Early Jurassic-Early Cretaceous (Aluk stage) was NW-SE. During this stage, subsidiary structures like deformation bands could have been developed together with the main dextral transcurrent faulting in the Huincul High, when the Lajas reservoirs were being buried. Contemporaneously, the source rock Los Molles Formation was in the oil window and the hydrocarbons with associated fluids (organic acids, CO2, water) may have migrated into the overlying Lajas Formation, favoring 1) grains dissolution in highly-porous dilation bands, 2) precipitation of siderite with hydrocarbon-bearing fluid inclusions, 3) kaolinite formation, and 4) bitumen impregnations. During the Upper Cretaceous (Farallón stage), the main stress field rotated to SSW-NNW, and the new tectonic regional trend, combined with an advanced burial state, may have caused a brittle behavior of Lajas Formation, as supported by the mode II/III fractures that cross-cut and displace the deformation bands.In summary, it is inferred that deformation bands in the Lajas reservoirs developed during the Aluk tectonic stage in the Huincul High, when the unit was not deeply buried. This study indicates that compaction bands act as barriers, whereas dilation bands act as fairways for hydrocarbons migration. Further studies at reservoir scale will be useful to define if these structures are clustered, hence effectively affecting fluid migration and accumulations, or if they are isolated features.