Important Reservoir Factors

o  Gross thickness •  The total thickness of the reservoir interval

o  Net-to-Gross ratio •  The fraction of the gross reservoir thickness

representing porous reservoir rocks

o  Porosity •  As fraction of the total net reservoir

o  Permeability •  Impacting the production rates that can be

achieved

o  Geometry of reservoir bodies: •  Affecting connectivity, and influences the

production rate & recovery factor 3 3. Reservoirs

What we like best:

Very thick

High net/gross massive sandstones

High porosity much room for HCs

High permeability high production rates

High continuity, and no “baffles”

Reservoir Factors are controlled by: o Depositional environment

From terrestrial to deep marine

o Rate of sediment input High sediment input Progradation Lack of sediment input Carbonate deposition

o Relative sea level Rise Regression and accumulation in accommodation space High Progradation into the basin Fall Non-deposition or erosion and by-pass to deep water Stable Lateral fill of accommodation space

N.B Steep slopes narrow shelves with fans & sediment by-pass

o Post-depositional structuration Faulting Differing subsidence rates (accommodation space) Uplift Erosion and unconformities

4 3. Reservoirs

Depositional Environments

5 3. Reservoirs

Clastic Depositional Systems

Clastic Depositional Systems

Clastic Depositional Systems

Sequence Stratigraphy for Dummies Sequence stratigraphy is the study of stratigraphic sequences in a

time framework linked to variations of (relative) sea level.

All cycles in sedimentation result from changes in accommodation space in a depositional setting.

Relative sea-level can be subdivided in 4 phases: Rising – High – Falling – Low

In the sedimentary record we can recognise 3 Systems Tracts: •  Transgressive Systems Tract – fast rising sea-level •  Highstand Systems Tract – slow rising (late) and high sea-level •  Low Stand Systems Tract – falling and low sea-level

Each phase may contain all facies from shallow to deep, but they are: •  spatially different, and •  have different onlap geometries

9

A very good website with information and illustrations about sequence stratigraphy can be found at:

http://www.sepmstrata.org/seqstrat.html 3. Reservoirs

Eustatic Sea-Level Cycles

JdJ 10

  1st Order Cycles   > 50-100 MY cycles   Resulting from large scale volume changes of

ocean basins related to break-up and formation of supercontinents

  2nd Order Cycles   3-50 (10-100) MY   Probably caused by changes in oceanic spreading rates

  3rd Order Cycles   0.5-3 (1-10) MY   Foundation of Seismic Stratigraphy depositional sequences   Possibly caused by glacio-eustatic cycles and/or variations in intra-plate stresses

  4th and higher Order Cycles   <0.5 MY   Local autocyclic causes, such as delta switching   Milankovitch cycles (Precession: 19-26ky, Obliquity: 41ky, Eccentricity: 95, 125,

400ky)

Sealevel Phanerozoic coastal onlap curve

Vail onlap curve

First-order sea-level cycle

The light blue line is the 1st order sea-level curve

The coastal onlap curve has 2nd order cyclicity Note the flat tops of the coastal onlap curve…

11 3. Reservoirs

Facies change within a depositional sequence Four main parameters influence distribution of facies

belts: 1.  Eustacy 2.  Subsidence 3.  Sediment supply 4.  Climate

After Bally et al. JdJ 12

Climate

Eustacy

Subsidence

Sediment supply

Eustacy, Relative Sea-level, Coastal Onlap

A symmetric sea-level curve results in a a-symmetric coastal onlap curve

13

Coastal onlap curve

Stratigraphic Recorded time

B/C D/E F/G

hiatus

hiatus

hiatus

Summation of input functions

time A B D E F G C

Relative changes

of sea-level R

ising Falling

Input functions

time

eustacy

Relative changes

of sea-level R

ising Falling

3. Reservoirs

Changes in sediment supply rate

JdJ 14

Coastal Onlap & Transgression / Regression

Rising sea-level Falling sea-level

Time of sea-level fall

Stillstand

Transgression

Regression

High sediment input

Low sediment input

After B.W. Ross 15

Fast sea-level rise

Slow sea-level rise

3. Reservoirs

Prograding depositional sequence

Coarsening upward progradational (regressive) sequence of fan-delta complex: •  Marly shales at the base (slope deposits) •  Coarsening/shallowing upward sands (coastal-fluvial/alluvial)

Arrow indicates shaling-out sand unit in distal direction: clear example of facies change within a single depositional sequence

16 3. Reservoirs