Taylor, Thomas R.1, Richard J. Stancliffe2, Calum
Macaulay1, Lori Hathon1
(1) Shell International E & P, Houston, TX
(2) Shell Offshore, Inc, New Orleans, LA
ABSTRACT: High Temperature Quartz Cementation and the Timing of Hydrocarbon Accumulation in the Jurassic Norphlet Sandstone, Offshore Gulf of Mexico, U.S.A
Jurassic Norphlet Formation sandstone reservoirs in Mobile Bay (offshore Alabama, U.S.A.) produce gas from great depths (> 20,000 ft) and elevated temperatures (>200ºC). Quartz cement is concentrated at the top of the reservoirs forming a low porosity “tight-zone” of variable thickness (10 – 190 ft) above a porous reservoir section. The extent of the tight-zone is independent of depositional facies and its thickness strongly influences well performance. Intergranular porosity in the Norphlet has been preserved by inhibition of quartz cementation due to the occurrence of robust grain-coating chlorite. Quantitative petrographic data reveal that grain-coat coverage is less in the tight-zone sands (mean=92%) than in the reservoir sands (mean=99%). Burial history and quartz precipitation kinetics modeling indicate that this seemingly minor difference in the completeness of grain coatings is sufficient to produce the observed differences in cementation and porosity. Quartz cementation in the tight-zone occurred under conditions of deep burial and high temperature. It followed the onset of pressure solution, emplacement of liquid hydrocarbons, and deposition of solid hydrocarbon (pyrobitumen) in the pore system. Fluid inclusion data indicate that volumetrically significant quartz cement precipitated at >150ºC from highly saline aqueous fluids. Hydrocarbon-bearing fluid inclusions are notably absent in quartz cement of the tight-zone. Oil and gas associated with pyrobitumen evidently escaped from Norphlet traps prior to tight-zone cementation. Gas presently found in Norphlet reservoirs of Mobile Bay represents a relatively recent accumulation and is not the product of in situ thermal cracking of oil.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.