REEFS AND SALTS

PART I: REEFS

I. Reef Characteristics:

A. Ecological and Paleoenvironmental Conditions

1. shallow water to allow for photosynthesis
2. agitation to oxygenate water
3. low turbidity and clastic supply
4. sufficient nutrients
5. sea surface temperatures above 18 C

B. Geology of Reefs

1. high porosity
2. high and permeability
3. increases in porosity with dolomitization because limestone volume reduced 12-13%.
4. resistant to post-depositional compression relative to the surrounding facies
5. sediment facies and ecological zonation with respect to the reef basin-marginal slope-reefal slope-reef margin- reef flat- back reef

C. Types of Reefs

1. fringing
2. barrier
3. atoll

II. Requirements for Hydrocarbon Accumulation:
A. source beds for petroleum
B. a reservoir
C. a suitable structural setting and a cap rock to "trap" petroleum

Why did reefs and their surrounding environments so often become sites of hydrocarbon accumulation?

III. Major Paleozoic Reef Formers
A. Earliest "Reef" Formers of Cambrian (Not true reefs but mound builders)

Stromatolites
Archeocyathids
Bryozoa and
Brachiopods

B. Reef formers of Ordovician-Devonian

1. Anthozoa- class of Cnideria

a. Tabulata - order, colonial
b. Rugosa -order, solitary or colonial, most solitary cup-corals Rugose or tetracorals (septa in quadrants)

2. Porifera (sponges) Stromatoporoids- large mound builders common in reef core
3. Bryozoans
4. Brachiopods
5. Crinoids

IV. Paleozoic Case Histories (to be discussed and illustrated in class)

A. Silurian Reefs of N.Y., Michigan, Ohio, Kentucky, Indiana, Wisconsin.

Middle Silurian barrier reefs circle the Michigan and Illinois Basins, another middle to late Silurian barrier reef in southern Illinois and Indiana.

Patch reefs proliferated throughout e. Iowa, se. Wisconsin, Illinois, Indiana, Ohio and Michigan.

Late Silurian barrier reefs on margins of Michigan, Ohio and New York Basins

B. Devonian Reefs of Williston Basin:

atolls to 40 miles across
Mid-Dev : reef growth closed by evaporites: halite, gypsum, anhydrite and potash salt
Upper Dev: Thick evaporites deposited in the basin on top of Elk Point group but a system of barrier and patch reefs grew on Alberta shelf. - These reefs a major source of oil. - Seen in outcrop of Front Range and in subsurface. - Two environments: (1) edge of platform which underlies S. Alberta (2) low ridges extending north into the basin from the platform -Types of reefs: (1) platform edge discontinuous barrier reefs (2) patch reefs in basin both 1 and 2 with corals, algae, stromatoporoids - to south of platform was a lagoon in N&S Dakota and Wyoming

A major calcareous shale basin to north of South Alberta reef complex. Within it were long lines of reef trends - the largest is between Calgary and Edmonton where Canadian oil boom began in 1947.

- Oil in porous reef dolomite, sealed by shales of basinal facies. Most reefs 100' thick, but north of Edmonton > 1000'.

- basin facies extends to Arctic Circle where production occurs from reefs in largely shale facies (MacKenzie R.)

Early Miss. - Madison Sea of western stable interior (Mission Canyon and Livingston Formations of NW states and Alberta). crinoidal limestones and calcarenites Livingston Fm - major cliff formers of Canadian Rockies 2000' in Cordillera 1000' in Front Range 500' in Williston Basin

Mission Canyon leached (increasing porosity) in Williston Basin. Buried by evaporites and Mesozoic shales. Major oil producer in S. Saskatchewan.

- near Front Range, folded and faulted by Laramide orogeny. In late Mississippian, the stable interior was uplifted, interior basins silted up. End of late Ordo - mid Mississippian epeiric seas.

PART II: EVAPORITES

Seawater contains about 3.5% dissolved salts as shown in Table 1.

TABLE 1 PROPORTIONS OF PRINCIPAL SALTS DISSOLVED IN NORMAL SEA WATER

SALTS, AMOUNT DISSOLVED, % THICKNESS IF PRECIPITATED FROM 2,000 m of WATER

NaCl (salt)	2.72	47.5
MgCl₂	.38	5.8
MgSO₄	.16	3.9
CaSO₄ (anhydrite)	.12	2.3
K₂SO₄	.08
CaCO₃ (calcite)	.01	0.6
MgBr₂	.01
	3.48	60.1 m

Expected to form in reverse order of their relative solubilities. This not always the case because of changes in basin inflow, dilution of brines, etc.

How much water evaporated to form 750 m of Michigan Basin evaporites?
Equivalent to a sea water column 1000 km deep! Alternatively, reflux in a shallow basin.

TABLE 2 Common Marine Evaporite Minerals

gypsum CaSO₄ * H₂0
halite NaCl
polyhalite K₂MgCa₂(SO₄)₄ * 2H₂O
sylvite KCl
carnalite KMgCl₃ * 6H₂O

anhydrite CaSO₄

NOTE: Anhydrite is dehydrated gypsum. Conversion of gypsum to anhydrite occurs with burial and loss of water which can result in a reduction of solid volume by 38%.

Example: If original gypsum had a porosity of 60% and gypsum constituted 80% of the sediment, after conversion to anhydrite it would have only 40% of the rocks original volume. This would result in basin subsidence.

NOTE: Bulk dissolution of evaporite by groundwater is very common. This process may cause collapse structures to form (e.g. New Mexico).

TABLE 3.

Locations, Thicknesses, Types and Ages of some North American Paleozoic Evaporites.

Location	Age	Thickness	Type
Williston Basin	Ordovician	25	S
	Sil. or Dev.	25	S
	Devonian	1000	S,H,B
	Mississippian	1000	S
Illinois Basin	Ordovician	50	S
	Mississippian	200	S
Michigan Basin	Sil. or Dev.	3000	S,H
	Devonian	1200	H
	Mississippian	350	H
Paradox Basin, Co	Permian	4000	S,H,B
Gypsum Basin, Co	Permian	500	S
N. New Mexico	Permian	100	S
S. central N. M.	Permian	3000	S,H
S.E. New Mexico	Permian	4500	S,H,B
Central Texas	Permian	1500	S
Texas Panhandle	Permian	2000	S
Western Oklahoma	Permian	1500	S
W. central Kansas	Permian	800	S,H
Iowa	Permian	50	S

S= gypsum and anhrdrite H= halite B= bedded bittern salts of potassium and magnesium