Abstract: Diagenesis in the Upper Lotena Formation, Late Jurassic, Neuquen Basin, Argentina

Meissinger, Viviana - YPF and María L. Rodríguez Schelotto - LCV s.r.l.

Cutting and core samples of 23 wells were lithologically and diagenetically analyzed in order to establish the reservoir potential conditions in the Loma del Mojón, Sierras Blancas exploration and Loma La Lata - Sierra Barrosa exploitation blocks. The study included description and quantification of more than 150 core and cutting thin sections, standard petrophysical analyses (porosity, permeability and grain density), X-ray diffractometry and scanning electron microscopy.

Core Selley profiles at 1:40 and cutting lithological descriptions at 1:200 and 1:1000 were drawn. The following parameters were taken in account: color, grain size, sorting, roundness, cement, grain coatings, porosity and hydrocarbon and bitumen presence.

The depositional environment consists of marginal marine flood plains where fluvial processes dominate over marine ones. Braided fluvial channel, distributary channel and interdistributary bay deposits were recognized on cores.

The flood-plain and deltaic feldspathic litharenites of the Upper Lotena Formation display a variety of diagenetic features. Fig. 1 corresponds to the main cements occurrence. Gray dots indicate wells with microscopic analyses. Authigenic chlorite has cemented the arenites in varying degree, ranging from 0 to 4%. The authigenic chlorite began to form early in the diagenetic history of the arenites ard constitutes clay rims on grains inhibiting subsequent secondary overgrowth quartz cementation. Fig. 2, SEM microphotograph shows a detailed view of early chlorite cement, note preserved intergranular porosity (IG). Prosecutors of chlorite are thought to be syndepositional flocculated clays due to mixing of fresh clay bearing with brackish waters.

The main features seeming to be crucial in the poral space preservation are the presence of chlorite rims and the degree of compaction. Grain percentage measured on thin sections was taken as an acceptable index of mechanical compaction. Chlorite rims were quite easily recognized in the wells during sandstone observation of cuttings and cores under the stereomicroscope.

Cross-sections illustrate the influence of diagenetic features on the porosity development and its relationship with the hydrocarbon emplacement. SW-NE section clearly shows mechanical compaction increasing towards the southwest.

Maps of grain percentage and presence of chlorite showed coincidence of low mechanical compaction and chlorite rims occurrence with the highest porosity values and together with lithofacial and potential reservoir thickness maps allowed to identify facies with the best reservoir quality.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil