Regional Stratigraphy and Sedimentology of the Eagle · PDF fileRegional Stratigraphy and...
Transcript of Regional Stratigraphy and Sedimentology of the Eagle · PDF fileRegional Stratigraphy and...
Regional Stratigraphy and Sedimentology of the Eagle Ford
Formation
Roger J. Barnaby
(My work was presented at AAPG Annual Meeting 2012, AAPG Black Shale Core Workshop 2013, and AAPG Memoir 102)
Eagle Ford Lithologies
• Limestones > 65% CaCO3 • Dunham texture, major grains
• Marls < 35% clay, 35-65% CaCO3 • Subdivided based on skeletal abundance
• Calcareous Shales > 35% clay
Limestone
Marl
Shale
Co
Py Si
Ca
Co
Py
Fm
Py
Ca
Co
Grain Types and Fossils
Foraminifera
Mollusks Pelecypods Inoceramids
Coccoliths Ammonite
Siliciclastic silt Phosphate grain
1 cm
Marl rip-up clasts
Pe
Fecal Pellets
Fish vertebrae Reworked volcanics
Inner Shelf to Lower Shoreface
• Limestone-rich
• Grain-rich, intraclast-skeletal PS/GS
• Ripple lamination, mud drapes, erosion surfaces, skeletal lags, rip-up clasts, diverse fauna
• Intense bioturbation
Inner Shelf to Lower Shoreface
1.0 mm
Fm
Ph
1.0 mm
Fm
Mo
Ph
OM
Grain-supported/ grain-rich texture
Core close-up views Petrographic microscope images
Ripple laminations
Mud drapes
1 cm
Erosion surfaces
Rip-up clasts
1 cm
Bioturbation
1 cm
Soft-sediment deformation
1 cm
Mid-Shelf Tempestites
• Cyclically interbedded marl and skeletal limestone
• Limestones exhibit ripple lamination, scours and gutter casts, skeletal lags, rip-ups
• Flat laminated marls represent pelagic suspension below SWB
• Skeletal packstones record storm reworking and winnowing
Mid-Shelf Tempestites
0.5 mm
Ph
Ph
In
Fm
In
Rip-up clast
1.0 mm
Fm
In
OM
Pe
Rip-up clasts
1 cm
Scour surfaces
1 cm
Ripple laminations
Skeletal lag 1 cm
Gutter cast
Core close-up views Petrographic microscope images
Outer Shelf Distal Tempestites
• Laminated marl, thin laminae of foram PS
• Flat- to lenticular-laminated skeletal PS display small-scale scours, starved ripples, and graded bedding
• Pelagic suspension, distal storms
Flat lamination
Inoceramids
1 cm
Starved ripples
Graded bed
Scour
1 cm
Slope/Basin Hemipelagics
• Laminated-massive marl with thin foram laminae
• Flat lamination to graded beds
• Small horizontal burrows
• Suspension and turbidity currents below SWB
Faint lamination 1 cm 1.0 mm
F
m
O
M
Bu
Ph
Persian Gulf: Eagle Ford Modern Analog
Craton
Orogenic Highlands
Orogenic Highlands
Craton
Persian Gulf, depth < 250 ft
Shamal winds transport clay and silt to basin center
Epeiric sea on convergent margin between stable craton and orogenic highlands
500 mi
Persian Gulf Analog
Photic Zone 60-90 ft Bioturbation < 120 ft Storm Wave Base 75-150 ft
Shallow asymmetrical basin, sediment distribution function of water depth and shamal-generated waves Purser (1973)
Progradational Facies Succession
Inner Shelf To Lower Shoreface
Mid- to Inner Shelf
Outer Shelf
20 10 0 FT
Vertical Stacking: Sequence Stratigraphy
GR 0 200
COREGR_api0 200
DEPTHFT
12750
12800
12850
12900
12950
BUDA
PS
PS
PS
PS
PS
PS PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS PS PS PS
PS
PS PS
PS
PS
PS
PS
PS
PS
Buda
K63 SB
K 65 MFS
K 69 SB
K 64 SB
K 72 SB
Conclusions
• Eagle Ford: shallow-water (< 100 m) tempestite setting
• Water depth and SWB exert primary control on lithofacies
• Distal facies = highest TOC, proximal facies = lowest TOC
• Cyclic marls and limestones = layered heterogeneous reservoir
• Facies linked to mineralogy, can be interpreted from logs
• Vertical stacking patterns define 3rd order sequence stratigraphy for regional correlation and mapping