Asphaltenic-Rich Tar in the Subsurface: Case Study From Reservoir Overburden in a Giant Subsalt Oil Field (Mad Dog, Gulf of Mexico, U.S.A.)

Abstract

Unanticipated tar occurrences in the subsurface can induce costly, undesired rig downtime without warning, particularly in deepwater settings such as when drilling through reservoir overburden of the Mad Dog oil field located in the Gulf of Mexico. These drilling hazards primarily include tar dikes and sills in uppermost Miocene sediments just below the salt canopy. Historically, over a third of Mad Dog related rig downtime has been caused by issues related to drilling through tar. As such characterizing and categorizing the geological and geochemical mechanisms responsible for the presence of each natural asphaltene deposition is critical to containing drilling costs. Communicating subsequent findings with development engineers and geologists in a common language that improves our understanding of tar and petroleum system impacts allow potential problems to be minimized from development plans. Here we show all sampled tar occurrences in the Mad Dog structure to be comprised of early expelled products from the source rock of much lower maturity than the fluids present in the main pay. Tar exhibits higher asphaltenic and sulfur contents than are present in the main pay reservoir fluids. Charge budget and timing constraints, as derived through map-based basin modeling workflows, provide the understanding low mature fluids arrived at the reservoir before the main reservoir seal became effective, enabling the initial expelled products from the source rock to migrate vertically through the main reservoir and likely all the way to the paleo-mudline, spreading out laterally on the seabed. The vast majority of the charge budget didn’t arrive to the reservoir until afterwards when the seal became effective at ~6 ma, allowing the vast majority of the charge budget to be captured during the Pliocene and Pleistocene, which in turn no longer enabled tar to migrate and accumulate in shallower reservoir overburden sediments.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019