Print this pageCharacterization of Tars in Deep Water Gulf of Mexico: Applications to Drilling and Understanding Petroleum System Mechanisms
In the deepwater Gulf of Mexico, drilling operations have encountered tar-like materials on a regional and field scale. This tar is often associated with faults, fractures, unconformities, and salt features above the oil columns; sometimes it occurs in reservoir zones. These tars are often solid at the surface but display a range of mobility in the subsurface resulting in a variable abundance and extrusion into the wellbore. Numerous terms have been used to describe these tars, which imply specific properties. Reconstructing the mobilization and remobilization of tar, and predicting its occurrence associated with a well-path is difficult. Tar can impact drilling operations, completion of well objectives, and affect field development.
Characterization of these tars defines their origin, physio-chemical properties, and helps provide insight into their occurrence and drilling response. Most tars are derived from an organic-rich, carbonate-dominated Mesozoic source generally similar to the regionally defined oil family. In individual fields, small but distinct differences are often identified between tars and associated oils. Tars have a similar or relatively lower thermal maturity than the associated oils. The tar is often derived from a source facies enriched in bacterial input resulting in higher nitrogen, oxygen and sulfur compared to the corresponding oils. This facilitates abundant generation of bitumen in the early oil window that is rich in heteroatoms and asphaltenes. The combination of its marginal maturity, low paraffin content, non-biodegradation, high viscosity, and unique occurrence suggests many of these tars represent an early expelled product. Other origins of some of these tars may include: 1) biodegradation or alteration products, 2) asphaltic precipitation, and 3) oil impregnated sands. Unraveling the effects of mud additives is important in their characterization. Basin modeling helps define generation/expulsion and initial compositional properties. Comparison of these tars with global analogs identifies similarities and distinctions in their origin, occurrence and properties. The characterization of tar integrated with geological and regional geochemical information helps define mechanisms and dynamics of the petroleum system. Utilization of the physio-chemical properties of tar combined with how it is encountered in the wellbore can help ascertain the drilling related response and contribute to a tar mitigation plan.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009