Abstract: Geochemical Study of the Upper Jurassic Source Rocks and Oils In Tampico-Misantla Basin (Mexico)

Bernal-Vargas, L.; Mena-Sánchez, E.; Román-Ramos, J. R.; and Cuevas-Leree, J. A. - Pemex

Tampico-Misantla Basin (TMB) is located in central-eastern Mexico between Sierra Madre Oriental Fold Belt (SMOFB) and Gulf of Mexico to the east. The intrabasinal basement high Tamaulipas-Tuxpan-Las Hayas divides TMB into a western onshore portion and an eastern offshore portion ( 1). The stratigraphic column comprises rocks from Lower Jurassic to Tertiary ( 2). Almost 100 Mbbld of oil and gas associated are produced from Kimmeridgian and Cretaceous limestones, and Eocene sandstones. This study is focused to establish the correlation between source rocks and hydrocarbons to approach the quantity and quality of generated and migrated hydrocarbons.

Geochemical data suggests the main sources be related with upper Jurassic formations. The thickness and richness are controlled by paleogeography. The richest sources were deposited in subbasins. Santiago formation consists of Oxfordian black carbonate shales. Its thickness varies from 10 to 755 m. Carbon organic contents (TOC) range from 0.5 to 6.3%, Hydrogen Index (HI) varies from 21 to 1079 and the Tmax- from 425 to 525°C. Biomarkers indicate a marine hypersaline shaly suboxic environment. Taman formation consists of Kimmeridgian shaly mudstone interbeded with laminar carbonate shales. Its thickness varies from 3 to 988 m. TOC range from 0.1 to 5.4%, HI varies from 15 to 925 and the Tmax- from 421 to 527°C. Biomarkers suggests a marine hypersaline carbonate anoxic environment (Hopanes: C29>C30, C34<C35, low Diasteranes/Steranes ratio). Pimienta formation consists of Tithonian thick black mudstone interbeded with laminar black shales. Its thickness varies from 3 to 485 m. TOC range from 0.4 to 6.5%, HI varies from 18 to 959 and the Tmax- from 412 to 476°C. Biomarkers suggests a marine hypersaline shaly carbonate anoxic environment (Hopanes: C29>>C30, C34<C35, fair Diasteranes/Steranes ratio). Therefore, Oxfordian and Tithonian sources are the organic richest, containing mainly a mature type I/II oil prone kerogen. We delimited 13 paleokitchens ( 1). Average paleokitchen area and thickness are 907 Km² and 550 m, respectively. Average TOC and HI are 2.2% and 500, respectively onshore. Transformation ratio is 45%. The initial hydrocarbon charge reaches 13 ton/m².

Upper Jurassic and Cretaceous reservoirs produce heavy oils (10-28°API) with high sulfur content (1.5-4.3%). Middle cretaceous rocks, offshore, produce light oils (>35°API). Tertiary fields produce oils with a density range from 25 to 35°API. Cretaceous heavy oils chromatograms have a hump base line that indicates biodegradation. These oils occur on eroded areas during tertiary. Jurassic heavy oils and Middle cretaceous light oils are related with kerogen maturity of expulsion. Tertiary light oils are related with segregation. Oil maturity has a good correlation with Ts/(Ts+Tm) hopanes ratio. Biomarker and isotopic oil composition suggests three families: anoxic marine shaly carbonate oils (onshore), anoxic marine carbonate (offshore) and suboxic marine shaly oils. Jurassic oils have a good correlation with Oxfordian and Kimmeridgian source rocks. Other oils are the mixes of the hydrocarbons expulsed by all Jurassic source rocks.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil