DIAGENESIS OF THE SANDSTONES OF A TIGHT-GAS RESERVOIR: THE LAJAS FORMATION IN THE HUINCUL HIGH, NEUQUÉN BASIN

RAINOLDI, ANA LAURA; BEAUFORT DANIEL; FRANCHINI, MARTA; MINISINI, DANIEL; PATRIER PATRICIA; GIUSIANO, ADOLFO; PONS, JOSEFINA; CESARETTI, NORA NOEMÍ

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

The sandstones of the Lajas Formation represent the main tight-gas reservoir of the Neuquén Basin. Diagenesis plays a fundamental role in this type of reservoir because diagenetic minerals have a high impact on porosity and permeability. This study presents the diagenetic characterization of the Lajas sandstones located in the productive area of Sierra Barrosa, central part of the Huincul High in the Neuquén Basin. The objective is to evaluate diagenetic processes and their impact on reservoir quality.The facies analyzed belong to deltaic and fluvial depositional systems with a variety of sub-settings including delta front (proximal and distal mouth bar), prodelta, delta plain (distributary channels and interdistributary bays), fluvial channel infill and fluvial plain deposits. The analyzed samples consist of very coarse-grained to fine-grained, medium- to poorly-sorted and texturally immature feldspathic litharenites and lithic feldarenites (sensu Folk et al., 1970) along with siltstones and subordinate fine-grained clast- and matrix-supported polimictic conglomerates. Detrital minerals include lithic fragments, mono- and polycrystalline quartz, and feldspar with K-feldspar>plagioclase. Lithic fragments are volcanic and volcanoclastic detritus and subordinate plutonic, sedimentary and metamorphic grains. The amount of cement varies considerably and includes pyrite, clays, quartz, feldspar, carbonates, barite and subordinated Zn-Pb sulfides. Porosity depends on facies and their amount of cements.The analyses indicate that, during early diagenesis, framboidal pyrite (Py1) precipitated as a result of bacterial sulfate reduction of aqueous sulfate. Framboidal pyrite occurs in fine-grained sandstones and siltstones in bioturbation traces and in phytoclasts, as well as disseminated in coarse-grained sandstones. Cubic pyrite crystals are commonly associated with framboidal pyrite suggesting recrystallization of earlier framboids. Kaolinite and berthierine were formed during eodiagenesis. Crystal size, blocky habit and composition of kaolinite close to ideal stoichiometry (Si2Al2O5 (OH4)) support a diagenetic origin. It is inferred that, with increasing burial, berthierine was progressively converted to Fe-rich chlorite (according to the intensity of d001