Mass-transport deposits are sedimentary, stratigraphic successions that were remobilized after initial deposition but prior to substantial lithification and transported downslope by gravitational processes as non-Newtonian rheological units (Bingham plastics or dilatant fluids). The deposits are not associated with a specific stratigraphic position. Mass transport complex (MTC) reservoirs currently under study in Colombia, consist of complex mixtures of metamorphic and igneous rocks with matrix porosities in the 3%-6% range, complex mineralogy, strong localized mylonitization effect and naturally fractured. The reservoirs are in the Lower Magdalena Valley hydrocarbon province in northern Colombia. In this context, the objective is to achieve an accurate porosity determination, water, gas, oil saturation computation and natural fracture assessment in an exploration phase, with scarce background on electrical logs response and petrophysical models in these types of environments.

In wildcat conditions, the lack of formation properties knowledge is detrimental to achieve a representative formation evaluation and reservoir potential understanding. While this is true even in relatively known geological environments (siliciclastics, carbonates, for instance), in Mass Transports deposits the complexity is even higher, given the mineral mixtures, heterogeneities, poor rock quality, complex tortuosity and complex natural fractures networks, among other challenges.

In this paper, we present an open hole advanced formation evaluation approach that enable to assess the tight matrix and the natural fractures systems, at a level not previously accomplished in these types of geological formations. At the matrix, advanced nuclear spectroscopy that does simultaneous time and energy domain measurements was integrated with a high-resolution magnetic resonance with improved sensitivity at short relaxation times. This allowed an archie-independent methodology for assessing gas from a new Fast Neutron Cross Section measurement, followed by liquid hydrocarbon fraction from the total organic carbon log and matrix-corrected porosity from combining hydrogen index and dry weight elemental concentrations. For natural fractures, the integration of borehole images with radial sonic-based dispersion and stoneley analysis, was carried out.

The main advantages of the new method for obtaining porosity, mineralogy, archie-independent hydrocarbon saturation in tight matrix and natural fracture assessment are: 1) conversion of fast neutron cross section to gas saturation and dry weight total carbon to oil saturation, done through a simultaneous inversion by solving matrix-porosity-fluids volumes into an elemental analysis, proven to work at low porosities; 2) reservoir quality assessment from a high sensitive and high resolution NMR T2 relaxation; 3) independency of archie equation parameters, typically unknow in wildcat environments; 4) reservoir potential uncertainty reduction; 5) identification of the natural fracture systems that can contribute to fluids production.

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