1. SCAL sample selection using core sedimentology integrated with hydraulic flow units
1 2
Ashish Banik SPE & Gordon Coy Ph.D.
1. Petroleum Development Oman, 2. Badley Ashton & Associates Ltd
The Precambrian to early Cambrian Ara Group intrasalt carbonates in the South Oman Salt Basin comprise at least six third-order evaporite carbonate depositional sequences (A1C-A6C). Reserves in the Birba
area (A4C) have been exploited since the late 70‘s and the Harweel area is currently being developed. A significant well database (core, openhole logs, NMR, BHI) is available for the Ara Group intrasalt
carbonates in both areas, although core coverage in individual reservoirs is more limited. Through detailed sedimentological evaluation of the cores and the application of sequence stratigraphic principles
(ongoing work by Badley Ashton & Associates Ltd geologists and the PDO Project Team), facies distributions within individual reservoirs can be predicted and static models built. In order to effectively populate
the static model with SCAL measurements and upscale the results for dynamic simulations, the SCAL programme needs to be closely tied to both the sedimentological and petrophysical characteristics of the
reservoir. In order to maximise the effectiveness of the SCAL programme, the dataset must cover the variations in both lithofacies and petrophysical properties. To achieve this, a representative suite of samples
is selected by integrating the following approaches.
Core sedimentology. The sedimentology of the cored Depositional model for the A2C Ara Intrasalt carbonates. Hydraulic flow units. Pore throat geometry, which KOZENY CORRELATION
intervals is described in terms of generic lithofacies, is controlled by depositional texture (lithofacies) and
based on depositional texture, which are grouped into LA3 [Bml] subsequent diagenetic alteration, is the primary
log (RQI) = log (PHIZ) + log (FZI)
lithofacies associations that represent more predictable LA2
control on fluid flow, wettability and saturation. By
e le m e n ts d e p o s it e d o n c a rb o n a t e ra m ps . T h e
LA6 [Ppst]
defining hydraulic flow units with internally consistent
where:
distribution of lithofacies associations within the LA3
LA8 [Mcl]
fluid flow properties the uncertainties in the lithofacies
reservoir, necessary for populating reservoir models, is LA7
based pore-system analysis can be reduced. The
[PG, WMp, Ppst]
Reservoir Quality Index (RQI)
predicted using a combination of sequence stratigraphic
LA4
hydraulic flow units are defined using the generalised
principles, openhole log responses and borehole image Kozeny-Carman equation for porous media. For
RQI (µm) = 0.0314*
in t e rp re ta t io n s . A lt h o u gh , p o s t d e p o s it io n t h e b ase samples with similar Flow Zone Indicator (FZI)
wa
ve
Sqrt(K[mD]/Ø[frac.])
sediments have undergone significant diagenetic lithofacies A
Salina Evaporites (2) fai
r
ath
er
w e e bas
wa
v
e
values, a log-log plot of the reservoir quality index
alteration, such as dolomitisation, cementation and Br Anhydrite breccia
sto
rm
lithofacies Mfl
(RQI) versus the pore volume-to-grain-volume ratio
A Anhydrite Slope/basinal
(FR) will yield a straight line with unit slope. Samples Pore volume to grain volume
leaching, which results in variable poroperm and pore M Mudstones
mudstones (8)
Mcl Crenulated microbial-
ratio (PHIZ)
bound mudstone
with different FZI values will lie on other parallel lines.
Mfl Finely laminated
system characteristics, the sedimentological model
lithofacies Bml(t) mudstone
Peritidal (3) Mtl Thickly laminated
mudstone
Mtb Thinly bedded
Samples that lie on the same straight line have
PGpi Pisolithic pack/grainstone
provides an important framework for detailed pore
mudstone
CR rhythmite mudstone
Carbonate
lithofacies BMth lithofacies WMP Mm Massive mudstone
Bml undifferentiated, smooth(s),
tufted and pustular Open marine Slope/basinal turbidites (7)
similar pore throat characteristics and, therefore, PHIZ = Ø/(1- Ø)
system analysis and the SCAL programme. SCAL thrombolites (4)
(s,t,p) microbial laminites
Massive thrombolitic PG(p, Peloidal(p), ooidal(o), intraclast(i)
Bthm boundstone o,i,c) or Claudina(c) pack/grainstone
Laminated thrombolitic Key
constitute a flow unit. Due to variations in diagenetic
Ppm Massive peloidal packstone
samples are coded by lithofacies and lithofacies
Bthl boundstone lithofacies PG (o)
Ppst Stratified peloidal packstone Lithofacies Association
BMth Thrombolitic bound/ Grainstone shoals or
Flow Zone Indicator (FZI)
mudstone Salt Pan
isolated sand sheets (5) WMp Peloidal wacke/mudstone
Salina
association, in order that they can be tied to the static overprint these flow units generally contain a range of
Brth Thrombolitic breccia
Peloidal(p), ooidal(o), Peritidal
PG(p, Cloudina skeletal(c), or Open marine thrombolites
o,c,n) Namacalathus (n) pack/
grainstone lithofacies Ppst Grainstone shoals or isolated
model, and the SCAL analyses distributed so as to give lithofacies, although groupings of distinct lithofacies
sand sheets
Offshore transition zone (6) Offshore transition zone
FZI = RQI/PHIZ
Slope / basinal turbidites
Ppm Massive peloidal packstone
within individual flow units are noted.
Slope / basinal mudstones
maximum coverage of the sedimentology. Ppst Stratified peloidal packstone
WMp Peloidal wacke/mudstone
1.) Detailed sedimentological description 4.) Use Kozeny correlation coefficients to define 5.) Compare poroperm relationships for facies asso- 6.) Asses distribution of potential SCAL samples
Association
CARBONATE TEXTURES
Lithofacies
Lithofacies
Zalzala-4
of the core hydraulic units for the core analysis dataset. ciations within individual hydraulic units. within lithofacies associations and Hydraulic units.
Lithology
Depth
(m dd) Plug type SCAL Orientation H Core type Conv.
1:400 M W P G F RBC
ALA-3 - 4941.80mlaminite
(Bml);
Microbial
dd C
LA-5 - Coarse-grained
peloidal grainstone
SEDIMENTARY STRUCTURES
SKELETAL ASSEMBLAGE
10.000
Hydraulic unit 5 Count of Plug
(PG); 4961.00m dd 4940 1000
A 4942 3 Bml
Ppst
5 6
Ppst
PG
100
4946
LA Code
16
karstification 2
4950
WMp
Ppm
14 3
6 10 4
KL(mD)
12
5
KL(md)
4954 10
6
1
8 7
8
Hy
WMp 2 6
1 9
4958
B
dra
3 4
4 Bthm
1.000 2
C
ulic
3 Bml karstification 4
5 PG 0
4962 4 Bthm 9
5
uni
0.1 2 3 8
5 PG 7
4 5 6
ociation
t
6 5
karstification
s Ass
6 Ppm 6 7 4
4966
Lithofacie
3
8 9 2
Unit 6 Yes
Unit 5 0.01 Hydraulic unit
Sponge core
4970 Unit 4 0.00 5.00 10.00 15.00 20.00 25.00
7.) Select samples for SCAL analysis to be
RQI
Unit 3 ØHe(%)
ØHe (%)
4974
Unit 2
Unit 1
Hydraulic unit 2 representative of both hydraulic units and lithofacies
10 associations, and to respect main reservoir units.
Plug type SCAL Orientation H Core type Conv. Suggested core plug selection (All)
5 PG Count of Plug
4978
6 Ppm
LA-4 - Laminated
B thrombolitic boundstone
(Bthl); 4959.30m dd
4982
4 Brth
0.100
Ppm
6 WMp 1
4986 5 PG
KL(mD)
Brth
LA Code
KL(md)
4990 4 8 2
Bthm 3
7
karstification 4
6
4994 1 5
6 Ppst 5 6
0.1
2 4 7
Hy
4 Bthm
3
8
4998
dra
3
6 Ppst 2
2 3
3 Bml
4 5
ulic
4 Bthm 4 1
6 7
0
5002 6 Ppst 8 9 5
uni
8
Yes
Bml
3 4 Bthm 7
0.010 6
iation
0.01
t
s Assoc
6 5
0.001 0.010 0.100 1.000 4
Lithofacie
WMp
5006 0.00 5.00 10.00 15.00 20.00 25.00
Mtl
7 8 3
PHIz ØHe(%)