CARBONATE SLOPE DEPOSITS; EVAPORITES; DOLOMITE AND DOLOMITIZATION

Attributes of Carbonate Slopes

• Dumping ground for shallow water sediments

• Sediments deposited by various sediment gravity flow processes

• Forms foundation for platform margin progradation

• Rare setting for carbonate reservoirs

Slope models for Ramp to Rimmed Shelf Transition from low slope to high slope gradient

foreshore-shoreface

gullied slope

build-up slope

rim-debris flow

Slope Deposits/ Elements and Position

Sediment Gravity Flows

sediment (blocks, grains, mud) • water • gravity

rock fall/avalanche

freefalling single blocks and angular talus

rock fall/avalanche

Rotational Slump

Shear failure along concave-up planes of rotation

Rotational Slump

Grain Flow

Flow support from cohesionless grain dispersive pressure (grain-to-grain contact

Grain Flow

Debris Flow

Flow support from matrix strength/cohesion and clast buoyancy

debris flow

Turbidites

Flow support from fluid turbulence

Turbidites

Depositional

shallow water reef/ shallow water lime sand shoals

Bypass

shallow water reef/ shallow water lime sand shoals

Rimmed

ramp

sediment dispersal pattern

point source rare; more common fine source from numerous shallow gullies in upper slope

slope angles

relatively high (avg.5-15) increases with height

potential for oversteepening

High- as slopes become steeper, they become more easily cemented in a marine environment

submarine cementation

common, allows oversteepening with resultant catastrophic collapse

slope profile

concave

constructions

base of slope aprons , slope aprons, next to rimmed or open platforms; rare submarine fans

slope types

ramp to rimmed shelf; by-pass to depositional

evaporites

•Controls on evaporite formation

•Environments/Facies Models

•Sedimentary Structures of Evaporite Deposits

Why are evaporites important?

classic evaporite systems as reservoir seals to world‘s large oil fields

increasing evaporation in brine concentration

caco3 - 1.8x

caso4 - 3.8x

nacl -10.6x

k salts - 70x

mud flat evaporites

formed by displacive evaporite growth in pre-excisting sediments

dry mud flat facies

evaporites non-accumulative, former presence revealed by modification to primary depositional and desiccation structure

saline water flat facies

evaporites persistent crystals; few sedimentary structures survive in host sediments

shallow water evaporites

formed by precipitation and/or sedimentation from shallow surface brine which are permanent or ephemeral features of the environment

deep water evaporites

precipitated or emplaced in brine generally deeper than 20-40 m. Characteristically they lack dissolution surfaces and bottom crusts.

Sabkha

supratidal marine-marginal setting, subaerial evaporites, halite only as crust, thin progradation wedge geometry

barred basin

deep-water, silled inlet subaqueous evaporites, large scale, geochemical profile well developed, marine geochemistry

dolomite problem

dolomite should precipitate regionally but this is rare becuz kinetic factors seem to be a critical issue

modern carbonates and dolomites

dolomite minerals in modern environments have been discovered relative recent 1950-1960 ( in between 30 degrees and 30 degrees- in the equator area)

requirements for models for dolomitization

1. source of mg

2. high mg/ca ratio

the dolomite ‘promise’

dolomite unit cell is smarter than calcite

then porosity increases due to the transformation of a rock with a loose atomic packing to a tighter packing

so many of the carbonate oil, and gas reservoirs are dolomitic

Replacement Textures (destructive)

Mimetic Dolomite Textures (preservation of the structure)

cements

• Simple rhombic rims

• Highly zoned cements

• Saddle dolomite rims

Dolomite Cements: Non-ferroan rim

limpid (clear) dolomite

seepage-reflux

motivations (carbonate reservoir example)

Slope and basin reservoirs are common in clastic systems

few existing reservoirs: cretaceous( Mexico and Italy), Permian(US) and tertiary(Indonesia)

slope and basin plays are viable bc

grainy porous carbonate rocks exist on the slope, at the slope and basin( reservoir)

stratigraphic architecture allows traps that can be enhanced by structure

nearby basinal deposits can be both source and top seal

what’s the biggest issue with carbonate reservoir

lateral seal and upslope leaking are the biggest issues

deposit type classification

debris deposits

grained- dominated deposits

mud-dominated deposits

spatial architecture

strike extensive aprons

discontinuous geobodies

dip-view stratal architecture

accretionary vs escarpment margins