Gustavo Murillo-Muñetón

Brine and hydrocarbon evolution during the filling of the Cantarell Oil Field (Gulf of Mexico)

The main oil reservoir in the Cantarell Field, offshore Campeche, consists of a dolomitized carbonate breccia with an ejecta seal on top, considered to have been formed during the Chicxulub impact event. Two different dolomitization events have been identified associated with the reservoir. The first generation (D1) is a bright-red luminescent saddle dolomite while the second generation is a minute, non-luminescent dolomite (D2). Brine fluid inclusions show an evolution from D1 to D2 to higher temperatures (from 80–120 to 100–120 jC) and salinities (from 2–8 to 6–8 wt.% eq. NaCl). Hydrocarbon-bearing fluid inclusions evolved from heavy oils to light oils in D1 (from core to rim), while in D2, all inclusions appear to be formed by heavy oil with an jAPI similar to the oil contained in the present reservoir. These facts suggest that the end of the dolomitization process was closely related with the beginning of the main accumulation of oil into the reservoir, displacing the aqueous fluids and precluding the precipitation of carbonates. D 2003 Elsevier Science B.V. All rights reserved.

The Cretaceous–Paleogene Boundary Chicxulub Impact: Its Effect on Carbonate Sedimentation on the Western Margin of the Yucatan Platform and Nearby Areas

Outcrops and offshore Campeche borehole data clearly document the presence of a carbonate facies succession, including calcareous breccia, on the western Yucatan Platform (Campeche Sound) and the Chiapas-Tabasco Platform. This carbonate sequence is associated with ejecta that contains altered glass, shocked minerals, and accretionary lapilli derived from the Chicxulub impact on the Yucatan Platform. The Cretaceous–Paleogene (K-Pg) boundary sedimentary succession is found at the El Guayal, Bochil, and Chilil outcrops of Tabasco and Chiapas, and offshore Campeche, 300–500 km (186–311 mi) west of the Chicxulub structure center. From base to top, this succession consists of four subunits: (1) carbonate breccia, 40–300 m (131–984 ft) thick, without ejecta; (2) fine- to medium-grained carbonate breccia, 10–20 m (33–66 ft) thick, mixed with sparse ejecta; and (3) siltstone, shale, and carbonate sand facies, 9–30 m (29–98 ft) thick, containing abundant ejecta (altered glass and shocked quartz). This unit culminates in a nearly pure clay layer (2 cm [0.7 in.] thick) with the well-known iridium anomaly at the top. Unit 4 is a conglomeratic breccia ranging from 10 to 20 m (33 to 66 ft) thick containing ejecta that is interbedded with or overlays subunit 3 (the ejecta layer) in some wells. Subunits 1, 2, and 3 are highly dolomitized in offshore Campeche, and the glass in subunit 3 is altered to clay minerals (smectite). Subunits 1 and 2 constitute hydrocarbon reservoir facies, whereas subunit 3 corresponds to the sealing layer of these reservoirs. Regionally, this sequence displays a gradational structure that represents a large debris flow followed by ballistic and clastic sedimentation with materials reworked by currents. Moreover, well logs, areal distribution, and stratigraphic relationships suggest that the thick K-Pg boundary sedimentary succession is a base-of-slope apron deposit. Based on the stratigraphy, sedimentology, and distribution of impact materials in the carbonate sedimentary succession, the following sequence of events can be inferred: megaseismic shaking that induced the collapse of the platform margin and produced the lower breccia facies (subunits 1 and 2); ballistic emplacement of ejected material (carbonate fragments, shocked minerals and glass) that supplied components to subunit 2 and formed the ejecta layer (subunit 3), the latter acting as the seal for Cantarell and neighboring oil fields; and reworking of the ejecta layer and coarser-grained carbonate fragments by the effect of one or more impact-generated tsunami waves to form a conglomeratic breccia (subunit 4) within subunit 3.

Determination of true bed thickness using folded bed model and borehole data

The true bed thickness (t) is the actual thickness of a given formation perpendicular to the bedding plane. The value of t depends on the angle and the direction of the dip of the measured formation, as well as the drift angle and azimuth of the borehole. The traditional methods to calculate the parameter t consider only the case of monoclinal beds but not the case of a folded bed, which will cause deviations when the bed dip on the top is different from that on the bottom. To avoid these deviations, this paper shows an approach to calculate the values of t using a folded bed model. The deviations for the monoclinal bed model are positively related to the bed dip, the dip difference and the deviated angle of the wells. A case study from the Cantarell oil field complex in the southern Gulf of Mexico (offshore Campeche) is used to test the folded bed method. The results indicate that this model can yield more uniform spatial change of the values of t, whereas the monoclinal bed model will overestimate the average value of t. Compared to the folded bed model, the maximum relative deviation of t from the monoclinal bed model reaches 22.3% and the maximum absolute deviation of t reaches 34.5 m.