Department of Geology and Geological Engineering - PowerPoint Presentation

Slide1

Department of Geology and Geological Engineering

Van

Tuyl

Lecture Series-

Spring 2016

4:00-5:00 p.m. in Berthoud Hall Room 241

Thursday

,

February 11,

2016

J. Rick Sarg

S

. Huang, M. Swierenga, K. Logan, C. Symcox, K.

Tanavsuu-Milkeviciene

, and J. Boak

Colorado School of

Mines, V

. P.

Wright, Cardiff

University and BG Group

“

Lacustrine Carbonates – Beaches, Microbes, and Chemical Precipitates – A Tale of Two Lakes, Green River and South Atlantic

Pre-Salt

”

Abstract

:

Carbonate deposits in ancient lake systems are products of the biotic and chemical environments in these lakes and can comprise significant hydrocarbon reservoirs within organic-rich lacustrine systems. The Green River (Eocene) carbonates are excellent process analogues for the pre-Salt reservoirs (Lower Cretaceous) in the South Atlantic region. Despite being deposited in very different tectonic regimes – post-orogenic foreland (Green River) and

syn

-rift (South Atlantic), the lake depositional processes and chemical controls on lake carbonates are similar. The Green River Formation of Colorado, Utah, and Wyoming provides insight into the South Atlantic lake basins, the most significant new hydrocarbon province discovered in the last decade.

Both the Green River and the South Atlantic lakes are organic-rich, alkaline lakes with pH values thought to be as high as 9-10. Co-variance of C and O stable isotopes in the Green River and lack of spring deposits (i.e., tufa and travertine) indicates a closed lake system dominated by surface inflow. The Green River lake is rich in Na, Ca, Mg, and HCO

3

resulting in precipitation of

nahcolite

,

trona

, and calcite and early replacement

ferroan

dolomite. The South Atlantic rift lakes are thought to be closed systems as well, and are rich in Ca, Mg and Si resulting in formation of

stevensite

, calcite

spherulites

, and replacement dolomite. The formation of dolomite in both lake systems contributes to enhanced reservoir quality.

Carbonate sedimentary environments common to both lake systems include beach and shallow littoral

bioclastic

coquinas and

ooid

grainstones

; and littoral to sublittoral microbial deposits. The Green River shoreline deposits range from dolomitic

intraclastic

packstone

to

oolitic

and ostracod

grainstones

deposited in meter-scale shallowing upward cycles. The South Atlantic shorelines are characterized by m to 10’s of m thick, molluscan lime

rudstones

and

grainstones

also deposited in shallowing upward cycles.

Interparticle

porosity dominates in both lake shorelines. Microbial deposits are common in both systems and comprise

stromatolitic

and

thrombolitic

biostromes

and

bioherms

. The Green River microbial-rich units are characterized by 1-3 m thick, deepening upward cycles that commonly begin with shallow littoral

intraclastic

rudstone

/

grainstone

and/or

oolitic

wackestones

to

grainstones

. These are overlain by

dendrolitic

and agglutinated stromatolites and

thrombolites

. Cycles are capped by fine-grained laminated stromatolites and transition upward into oil shale. These shallow littoral microbial deposits transition offshore into lower sublittoral to upper

profundal

fine-grained, laminated stromatolites. Intra-particle,

interparticle

,

fenestral

, and

vuggy

pore types are common in microbial and associated deposits resulting in excellent reservoir quality

.

In contrast to the Green River, late

syn

-rift South Atlantic carbonates are dominated by meters to 10’s of m thick dolomitic

spherulite

grainstones

.

Spherulitic

deposits have high porosity and permeability and are excellent reservoirs. Porosity is pseudo-

fenestral

and is interpreted to result from

stevensite

dissolution

.