Floodplain aggradation in a semi-arid endorheic basin setting, Altiplano, Bolivia

Abstract

The importance of gas as a driver of security of energy supply and the influence of the role it currently has and will continue having in the energy transition has stimulated the development of further research studies in unconventional sources of natural gas. Mature gas fields of coarse-grained fluvial reservoirs have already reached their production plateau and are in a production decline with high-water cuts.
The northwest gas province of the Netherlands contains low-permeable fine-grained fluvial deposits of which the reservoir architecture and connectivity have not yet been widely studied using the standard industry techniques. For this reason, the geomorphological and sedimentological characteristics of the Río Colorado fluvial system were used as a modern analogue to determine how the river avulsion process has generated the current fluvial topography and why its repetition has developed a vertical stacking pattern of thin but laterally extensive layers.
Differential Global Positioning System data, Google Earth Pro Imagery, and previous studies focused on the evolution analysis of river positions in this region, allowed the observation of multiple repetitions of avulsion river in an area of approximately 400 km2, describing the fluvial sediments on top of the lacustrine deposits and representing alluvial ridges with an average width of 1500 m. Moreover, a thickness variation between the fluvial and lacustrine deposits up to 2 meters was recognised over 25 km, and a very gentle gradient slope, from 0.02° at the apex of the fluvial fan to 0.0022° in the flattened section towards the Salar de Uyuni.
Borehole data from the West Netherlands Basin and surface data from the Río Colorado were integrated to construct a three-dimensional model that represents the characteristics of the fluvial deposits regarding geobody size, geometry, and potential connectivity. With this, and the distribution of porosity and permeability as a function of channel belt and alluvial ridge width resulted in reservoir volumes up to 2×〖10〗^9 sm^3.
Six wells were simulated to understand the production behaviour in channel belts, alluvial ridges, and floodplain sediments. Water saturation, pressure and streamline maps differ from well to well, and the model contributed to quantify the most efficient production rates per well location in the fluvial fan system. The results allowed to predict the communication between facies and how that connectivity reflected the resultant flow rates and recovery factors, showing the tough gas as potential secondary targets and prolonging the production stages of mature oil fields.