Abstract

The Talos Energy Zama-1SON, the first private-sector exploration well operated in Mexico in 78 years, was drilled in May–July 2017 in Block 7, 50 km (30 mi) offshore Tabasco in 166 m (545 ft) of water. The Zama structure was identified before the leasing round using two three-dimensional narrow-azimuth seismic data sets. The structure consists of an upthrown fault block with three-way closure on a salt structure’s flank in the eastern Salina del Istmo basin. A 344-m (1129-ft) gross sandstone reservoir interval was penetrated, containing 29.6° API oil. The estimated reserves make Zama one of the most significant offshore discoveries globally in several years.

An array of tools and techniques was used to define, drill, and evaluate the Zama prospect. These include (1) structural and stratigraphic analysis to frame the prospect in proper context; (2) predrill amplitude versus offset analysis calibrated to seismic data using existing well control; (3) petrophysical analysis using x-ray diffraction (XRD) mineralogy and image logs; (4) forward modeling for predrill stratigraphic control and reservoir thickness; (5) a full suite of logging while drilling (LWD) and wireline logs (including elemental spectroscopy, formation pressure testing, and fluid sampling); and (6) combined biostratigraphic and petro-logic (XRD and x-ray fluorescence [XRF] analyses performed on cuttings while drilling, drill stem test (DST)/pressure, volume, temperature (PVT) analysis, and whole-core analysis.

The reservoir section is dominated by amalgamated, coarse-to-very fine-grained, highly feldspathic, unconsolidated, poorly sorted sandstones with low clay content. Structural mapping and biostratigraphy suggest sediment may have been fed into an evolving late Miocene offshore basin from a narrow shelf and proximal alluvial fan complex in a very active geologic setting before being deposited in a deep-water environment as a submarine channel lobe complex. The base of the reservoir section coincides with a significant middle Miocene unconformity related to salt tectonics. Pressure gradient data confirmed only one hydrostatic system in the reservoir, and a pressurized fluid sample was acquired. Subsequent appraisal drilling included whole cores in two wells covering the entire reservoir interval and DST analysis to define the reservoir qualities and extent.