Handling a mature clastic HC reservoir without seismic - trends, facies proportions and depositional zones
Szabolcs Borka
University of Szeged, Institute of Geography and Geology, Department of Geology and Paleontology borka.szabolcs(5> gmail.com
One can use net-to-gross, lithological or depositional facies to build a static reservoir model.
Considering the latter, it is hard to reveal and put them back to our 3D model in the absence of seismic data because of their existing trends and geometries. Thus, we should rely on the well data to define and visualize depositional facies. This article provides a useful method to handle the apparent lateral "blindness".
The area of the case study is located in the southern part of the Great Hungarian Plain (Algyo field, Hungary). The formation is a submarine fan system. The reservoir is well explored by 132 wells.
Well-logs (GR, RES, SP, Sand- and shale-content, porosity etc.) were used to determine a simplified channel-lobe-background (depositional facies) system. Core samples from 7 wells were also available. In each well the facies proportions were calculated and encoded (0-1) corresponding to the dominant facies in the wells. After applying a simple local B-spline interpolation algorithm on these codes, the facies trends were unfolded as a property map.
The truncated Gaussian simulation was a suitable method to make this information spatially robust regarding facies trends (channel/proximal part or lobe/distal part) and to obtain facies zones. These facies zones were incorporated as trends into a sequential indicator simulation algorithm, thereby establishing a consequent 3D reservoir model. In two parts of the reservoir, channel-dominated zones can be recognized with the surrounding terminal lobes.
Key words: depositional facies, facies proportion, trend, truncated gaussian simulation, sequential indicator simulation, submarine fan
1. I N T R O D U C T I O N
A s t a t i c c l a s t i c - r e s e r v o i r f a c i e s m o d e l c a n b e b u i l t i n t h r e e d i f f e r e n t w a y s u s i n g n e t - t o - g r o s s , l i t h o l o g i c a l o r d e p o s i t i o n a l f a c i e s u n i t s . D u e t o i t s g o o d l a t e r a l i n f o r m a t i o n r i c h n e s s , s e i s m i c d a t a m a y a i d a l l t h e t h r e e m e t h o d s . I n t h e c a s e o f
t h e l a t t e r i t i s p e r h a p s t h e m o s t i m p o r t a n t s o f t d a t a b e c a u s e i t c a n s h o w g e o m e t r i e s a s s o c i a t e d w i t h d e p o s i t i o n a l f a c i e s . T h e r e f o r e , i n t h e a b s e n c e o f s e i s m i c , w e m a y f e e l l o s t r e g a r d i n g a n y k i n d o f t r e n d , e s p e c i a l l y i f t h e r e a r e h u g e a m o u n t o f w e l l d a t a w a i t i n g t o b e p r o c e s s e d . A p u r e i n t e r p o l a t e d a v e r a g e p o r o s i t y m a p c a n o f t e n h e l p t o o b t a i n t r e n d s , b u t t h e r e a r e o t h e r w a y s t o r e v e a l t h e m . F o r e x a m p l e , v i s u a l i z a t i o n o f f a c i e s p r o p o r t i o n s i n e a c h w e l l c a n s h o w a c l e a r p i c t u r e a b o u t t h e d i s t r i b u t i o n s o f f a c i e s w h i c h c a n b e u s e d a s t r e n d s o r g u i d e s f o r f a c i e s m o d e l l i n g . T h e a i m o f t h i s s t u d y i s t o p r o v i d e a w o r k f l o w f o r d e p o s i t i o n a l f a c i e s m o d e l l i n g b a s e d o n f a c i e s p r o p o r t i o n s .
T h e c a s e s t u d y i s l o c a t e d i n t h e s o u t h e r n p a r t o f t h e G r e a t H u n g a r i a n P l a i n ( A l g y o H C f i e l d , H u n g a r y ) . I t i s a s u b m a r i n e f a n s y s t e m p e n e t r a t e d b y 1 3 2 w e l l s w i t h a l l w e l l s p o s s e s s i n g t h e m a i n w e l l - l o g s ( S P , G R , R E S , D E N , C N , C A L , S a n d - a n d S h a l e - c o n t e n t , p o r o s i t y , p e r m e a b i l i t y ) . C o r e s a m p l e s f r o m 7 w e l l s w e r e a l s o a v a i l a b l e t o o b t a i n t h e d e p o s i t i o n a l f a c i e s .
T h e w h o l e w o r k f l o w w a s e x e c u t e d i n R o x a r ' s I R A P R M S p l a t f o r m ( a c a d e m i c l i c e n s e ) .
2. M E T H O D S
2 . 1 . Defining t h e d e p o s i t i o n a l f a c i e s in t h e w e l l s
T h e v e r y f i r s t s t e p w a s t o i d e n t i f y t h e s e d i m e n t a r y f a c i e s i n a s i m p l i f i e d c h a n n e l ( f i n i n g - u p w a r d s e c t i o n s ) - l o b e ( c o a r s e n i n g - u p w a r d s e c t i o n s ) - b a c k g r o u n d ( f i n e - g r a i n e d s e c t i o n s ) s y s t e m i n t h e 1 3 2 w e l l s b y u s i n g m a i n l y t h e G R , S P , Vs an d , V s h a i e , p o r o s i t y a n d p e r m e a b i l i t y l o g s . C o n s i d e r i n g t h e c o r e s a m p l e s , t h e c h a n n e l s ( F U s e q u e n c e s ) c o n s i s t o f t h i c k ( m a x : 1 0 m , m e a n : 4 . 5 m e t r e s ( f r o m d e f i n e d c h a n n e l f a c i e s i n t h e w e l l s ) m a s s i v e , s t r u c t u r e l e s s s a n d s t o n e s a n d t h e y a r e a l w a y s f o l l o w e d b y t h i n p a r a l l e l - , c r o s s - b e d d e d a n d / o r l e n t i c u l a r b e d d e d f i n e - g r a i n e d p a r t s . I n t h e c a s e o f s u b m a r i n e f a n s , t h i s k i n d o f s e q u e n c e r e f e r s t o a c h a n n e l - l e v e e s y s t e m . O n t h e o t h e r h a n d , l o b e s e c t i o n s ( C U s e q u e n c e s ) c a n b e d e s c r i b e d b y a l s o m a s s i v e s a n d s t o n e s , b u t t h e y a r e f o l l o w e d b y t h e s u d d e n a p p e a r a n c e o f s e p a r a t i n g s h a l e s ( m u d s t o n e s , a r g i l l a c e o u s m a r l s t o n e ) w i t h a n a v e r a g e t h i c k n e s s o f 4 m e t r e s ( m a x : 1 4 m ) . T h e b a c k g r o u n d f a c i e s i s m a s s i v e o r p a r a l l e l - b e d d e d m u d s t o n e s o r a r g i l l a c e o u s m a r l s t o n e s .
B a s e d o n t h e s e i n f o r m a t i o n , t h e c o n c e p t u a l g e o l o g i c a l m o d e l i s a m u d - r i c h s u b m a r i n e f a n s y s t e m w i t h m e a n d e r i n g c h a n n e l - l e v e e s y s t e m s ( m i d - f a n ) a n d t e r m i n a l l o b e s o r s a n d s h e e t s ( l o w e r - f a n ) a c c o r d i n g t o R i c h a r d s a n d B o w m a n ' s m o d e l ( 1 9 9 8 ) .
I t w a s q u i t e o b v i o u s t h a t i n t h e n o r t h e r n p a r t o f t h e r e s e r v o i r , b o t h c h a n n e l - l e v e e s y s t e m s a n d t e r m i n a l l o b e s w e r e p r e s e n t i n a g e n e r a l s e q u e n c e : t h e p a r t o f l o b e s a r e f o l l o w e d b y t h e c h a n n e l - l e v e e s y s t e m s . S o u t h w a r d s t h e l a t t e r g r a d u a l l y d e c r e a s e s i n t h i c k n e s s , i n t h e s o u t h e r n m o s t p a r t o f t h e r e s e r v o i r i t i s c o m p l e t e l y m i s s i n g ( p i n c h e s o u t ) , w h i l e t h e l o b e s a r e p r e s e n t i n t h e w h o l e a r e a o f t h e r e s e r v o i r . T h i s s u g g e s t s a p r o g r a d i n g s y s t e m t o w a r d s s o u t h ( F i g u r e 1 ) .
Figure 1: A prograding system seems to appear in the reservoir through 3 wells.
Note that southwards the channel-dominated parts (proximal parts) are absent, only lobe-dominated parts (distal parts) are present
2.2. E s t a b l i s h i n g f a c i e s proportion m a p s a s m a i n g u i d e s
A f t e r d e f i n i n g t h e d e p o s i t i o n a l f a c i e s i n t h e w e l l s , i t w a s p o s s i b l e t o c a l c u l a t e t h e f a c i e s p r o p o r t i o n s r e g a r d i n g e a c h f a c i e s ( c h a n n e l , l o b e a n d b a c k g r o u n d ) . F o r i n s t a n c e , i n t h e c a s e o f ' c h a n n e l f r a c t i o n d o m i n a n c e ' , i f t h e c h a n n e l f a c i e s
d o m i n a t e d r e l a t i v e l y o v e r t h e t w o o t h e r s i n a w e l l , t h e n t h a t w e l l w a s g i v e n a p a r a m e t e r ' 1 ' , e l s e a p a r a m e t e r ' 0 ' ( t h e c l a s s i c a l ' I F - T H A N - E L S E - E N D I F ' l o g i c a l e x p r e s s i o n ) . T h e s a m e m e t h o d w a s f o l l o w e d i n t h e m a t t e r s o f t h e l o b e a n d b a c k g r o u n d t o o . B y a p p l y i n g a s i m p l e l o c a l B - s p l i n e i n t e r p o l a t i o n m e t h o d o f t h e s e f a c i e s c o d e s , t h e e s t i m a t e d f a c i e s t r e n d p r o p e r t y m a p s w e r e u n f o l d e d . U s i n g t h e w e l l d a t a a n d t h e s e p r o p e r t y m a p s a s g u i d e s , t h e s o - c a l l e d ' p r o x i m a l ' ( c h a n n e l - l e v e e d o m i n a t e d ) a n d ' d i s t a l ' ( l o b e a n d s h e e t s d o m i n a t e d ) v a r i a b l e s w e r e d e f i n e d i n e v e r y w e l l ( F i g u r e 1)
2.3. C r e a t i n g d e p o s i t i o n a l z o n e s f o r p o s s i b l e 3 D t r e n d s a n d t h e final f a c i e s m o d e l
I n R o x a r ' s I r a p R M S i t i s p o s s i b l e t o u s e t r u n c a t e d G a u s s i a n s i m u l a t i o n ( ' F a c i e s B e l t s ' m o d u l e , T G S ) t o m o d e l d e p o s i t i o n a l z o n e s o r b e l t s , w h i c h i n t e r f i n g e r w i t h e a c h o t h e r . T h e r e s u l t s o f t h i s m o d u l e a r e o f t e n u s e d a s a d d i t i o n a l 3 D t r e n d s f o r t h e s e q u e n t i a l i n d i c a t o r s i m u l a t i o n ( S I S ) .
T h e d e f i n e d p r o x i m a l a n d d i s t a l f a c i e s w e r e s u i t a b l e i n p u t d a t a f o r t h e T G S t o r e v e a l 3 D t r e n d s f o r S I S . T w o m a i n p a r a m e t e r s w e r e n e e d e d : t h e d e p o s i t i o n a l d i r e c t i o n a n d t h e s o - c a l l e d s t a c k i n g a n g l e ( t h i s d e s c r i b e s t h e a v e r a g e a n g l e o f c l i m b o f t h e f a c i e s b e l t s w i t h i n t h e g r i d m o d e l ) . T h e d e p o s i t i o n a l d i r e c t i o n c o u l d b e p r e d i c t e d f r o m t h e t h i c k n e s s m a p o f t h e p r o x i m a l z o n e . F u r t h e r m o r e , i t w a s p o s s i b l e t o c a l c u l a t e d i p a n g l e s i n d e g r e e s ( f r o m n o d e t o n o d e , c o r r e s p o n d i n g t o t h e t h i c k n e s s v a l u e s ) b a s e d o n t h e t h i c k n e s s m a p o f t h e p r o x i m a l z o n e . T h e o r e t i c a l l y , i f a z o n e g r a d u a l l y b e c o m e s t h i n n e r s t r i c t l y i n o n e d i r e c t i o n
( a z i m u t h , i n c a s e o f t h e r e s e r v o i r , n o r t h t o s o u t h ) , t h e m e a n o f t h e s e a n g l e s r e p r e s e n t s a p p r o x i m a t e l y t h e m a i n s t a c k i n g a n g l e o f t h e z o n e , b e c a u s e b o t h t h e m e a n a n g l e a n d t h e d i p a n g l e o f t h e z o n e a s w e l l a s t h e m a i n s t a c k i n g a n g l e a r e a l t e r n a t e i n t e r i o r a n g l e s . T h e t h e o r e t i c a l b a c k g r o u n d c a n b e s e e n o n F i g u r e 2.
U s i n g t h e s e i n p u t p a r a m e t e r s , t h e T G S m e t h o d w a s e x e c u t e d ( w i t h a l t o g e t h e r 5 0 r e a l i z a t i o n s ) . T h e r e s u l t i n g P50 ( e v a l u a t e d f r o m a l l t h e r e a l i z a t i o n s ) 3 D m o d e l r e g a r d i n g t h e p r o x i m a l z o n e w a s i n c o r p o r a t e d i n t o t h e S I S a s a 3 D t r e n d f o r t h e c h a n n e l f a c i e s .
C r o s s s e c t i o n of t h i c k n e s s m a p of p r o x i m a l z o n e Calculated dip angles
from nodes to nodes
N
Distribution of these angles
$
Mean ~ Angle 'A' ft
A n g l e ' A = A n g l e ' B '
They are alternate interior angles
A /
Proximal ^ Distal
Stacking angle for TGS
N-S c r o s s - s e c t i o n of the r e s e r v o i r
Figure 2 : The theoretical background for calculating the main stacking angle in the case of the proximal zone which gradually becomes thinner strictly in one direction.
I t i s i m p o r t a n t , t h a t t h e r e s u l t o f t h e d i s t a l z o n e w a s n o t u s e d a s a t r e n d f o r t h e l o b e f a d e s , a s t h e r e s u l t p e r t a i n s t o b o t h t h e t e r m i n a l l o b e a n d t h e b a c k g r o u n d f a c i e s o n t h e e d g e o f t h e r e s e r v o i r , w h i c h l a c k s h a r d w e l l d a t a ( F i g u r e 1 ) . I n s t e a d , t h e m o d e l l i n g o f t h e l o b e a n d b a c k g r o u n d f a c i e s r e l i e d o n t h e i r g l o b a l f a c i e s p r o p o r t i o n s c o m i n g f r o m t h e u p s c a l e d w e l l d a t a a n d t h e i r s e m i v a r i o g r a m s , r e s p e c t i v e l y T h e g e o l o g i c a l i n t e r p r e t a t i o n o f t h e u s e d s e m i v a r i o g r a m s w a s n e c e s s a r y i n t h e c a s e o f e v e r y f a c i e s ( G r i n g a r t e n a n d D e u t s c h , 2 0 0 1 ) . T a b l e 1 s h o w s t h e t r e n d p a r a m e t e r s o f S I S f o r t h e d e p o s i t i o n a l f a c i e s m o d e l . U s i n g t h e S I S 1 0 0 r e a l i z a t i o n s w e r e g e n e r a t e d , a n d t h e f i n a l f a c i e s m o d e l w a s t h e P s o c a s e c o m p u t e d f r o m t h e d i s t r i b u t i o n o f a l l r e a l i z a t i o n s .
Table 1: The used trend parameters of depositional facies
Depositional facies Trend Semivariogram
Channel 3D parameter trend coming
from TGS 'proximal zone' Yes Lobe Global proportion coming from
upscaled well data Yes
Background Global proportion coming from
upscaled well data Yes
3. R E S U L T S
A f t e r c a l c u l a t i n g t h e f a c i e s f r a c t i o n s i n e a c h w e l l , i t w a s p o s s i b l e t o v i s u a l i z e t h e m a s p i e c h a r t s ( F i g u r e 3 ) . A s m e n t i o n e d b e f o r e , f o r o v e r v i e w i n g a m a t u r e
f i e l d i t m a y b e b e t t e r t o p a r a m e t r i z e r e g a r d i n g e a c h f a c i e s i n e v e r y w e l l . T h e p r o p e r t y m a p s o f t h e c h a n n e l a n d l o b e d o m i n a n c e c a n b e s e e n o n F i g u r e 3 .
Figure 3 : A: average porosity map with the facies pie charts, B: property map of channel dominance, C: property map of lobe dominance
I n t h e c a s e o f t h e c h a n n e l f a c i e s , t w o s h a r p z o n e s a r e r e c o g n i z a b l e f r o m n o r t h t o s o u t h , a n d f r o m n o r t h t o s o u t h e a s t .
U s i n g t h e s e i n f o r m a t i o n , t h e p r o x i m a l a n d d i s t a l p a r t s w e r e d e f i n e d i n a l l w e l l s . T h e t h i c k n e s s m a p s o f t h e d i s t a l a n d p r o x i m a l z o n e s a s w e l l a s t h e c a l c u l a t e d d i p a n g l e s r e g a r d i n g a f i l t e r e d p r o x i m a l ( e x c l u d e d t h e p a r t s w i t h z e r o t h i c k n e s s - > s e d i m e n t a r y p i n c h i n g o u t ) z o n e c a n b e s e e n o n F i g u r e 4.
A B C
Figure 4 : A: thickness map of distal zone, B: thickness map of proximal zone with the filtering polygon, C: dip angles of the filtered proximal zone
T h e d i s t r i b u t i o n p a r a m e t e r s o f t h e c a l c u l a t e d d i p a n g l e s o f t h e f i l t e r e d p r o x i m a l z o n e s ( m a i n s t a c k i n g a n g l e ) a r e : m e a n = 0 . 9 8 , m i n = 0 . 0 1 , m a x = 8 . 5 1 , s t d . d e v . = 0 . 7 9 . T h e d e p o s i t i o n a l d i r e c t i o n i s a b o u t 1 6 0 a z i m u t h d e g r e e ( s t d . d e v . = 1 0 ) . A f i l t e r i n g p o l y g o n w a s a l s o u s e d t o d e n o t e t h e f a c i e s b o r d e r b e t w e e n t h e p r o x i m a l a n d d i s t a l d e p o s i t i o n a l z o n e s i n T G S . A s m e n t i o n e d b e f o r e , o n l y t h e P s o o f t h e p r o x i m a l z o n e w a s p r o v i d e d a s a 3 D t r e n d f o r t h e S I S .
T a b l e 2 s h o w s t h e s e m i v a r i o g r a m s ' p a r a m e t e r s a n d i n t e r p r e t a t i o n s c o r r e s p o n d i n g t o e a c h f a c i e s i n t h e S I S .
Table 2: The semivariogram parameters and their interpretations (SV: subvariogram, in the case of a nested semivariogram structure; *range: parallel to azimuth/normal
to azimuth)
C h a n n e l S V 1 C h a n n e l S V 2 L o b e S V 1 L o b e S V 2 B a c k g r o u n d
T y p e S p h e r i c a l E x p o n e n t i a l N u g g e t E x p o n e n t i a l E x p o n e n t i a l
F r a c t i o n o f
s i l l 0 . 7 0 . 3 0 . 2 5 0 . 7 5 1
A z i m u t h 1 5 / 1 9 5 1 5 0 / 3 3 0 - 1 3 0 / 3 1 0 1 0 0
R a n g e * 7 0 0 / 5 0 0 3 5 0 0 / 1 6 0 0 - 3 9 0 0 / 2 8 0 0 2 5 0 0 / 1 0 0 0
I n t e r p r e t a t i o n
R a p i d c h a n g e s d u e t o t h e m e a n d e r i n g p r o c e s s i t s e l f ( p r o b a b l y
r e p r e s e n t s a w a v e l e n g t h )
t h e m a i n c h a n n e l d i r e c t i o n ( s m o o t h e r , p e r s i s t e n t c h a n g e )
-
t e r m i n a l p a r t o f t h e d e p o s i t i o n a l p r o c e s s e s b u t c o v e r s a g r e a t e r a r e a
t e r m i n a l p a r t o f t h e d e p o s i t i o n a l p r o c e s s e s b u t
c o v e r s a g r e a t e r a r e a
A l a y e r ( l a y e r 4 ) o f t h e f i n a l P50 m o d e l o f t h e r e s e r v o i r u s i n g S I S ( e v a l u a t e d f r o m t h e 1 0 0 r e a l i z a t i o n s ) c a n b e s e e n o n F i g u r e 5, c o m p a r e d t o t h e s a m e l a y e r o f r e s u l t o f T G S .
I t i s a p p a r e n t t h a t t h e m e t h o d p e r f o r m e d b e t t e r a t r e v e a l i n g t h e c h a n n e l c o m p a r e d t o t h e r e p r o d u c t i o n o f l o b e f e a t u r e s , p e r h a p s d u e t o t h e l a c k o f a 3 D t r e n d o f t h e l o b e i n t h e S I S .
Figure 5 : A: P5o result of SIS, layer 4; B: P5o result of TGS, layer 4; Note that in the case of 'A', two well visible main channels are recognizable
4. C O N C L U S I O N S
B a s e d o n t h e r e s u l t s , i t c a n b e s t a t e d t h a t i n c o n t r a s t w i t h t h e c o m m o n a s s u m p t i o n t h a t p i x e l - b a s e d ( e . g . S I S , T G S ) m e t h o d s a r e n o t a b l e t o r e p r o d u c e g e o m e t r i c a l i n f o r m a t i o n o f ( d e p o s i t i o n a l ) f a c i e s ( P y r c z a n d D e u t s c h , 2 0 1 4 ) , t h e u s e o f f a c i e s f r a c t i o n m a p s a n d d e p o s i t i o n a l z o n e s c a n a i d t h e s e m e t h o d s t o h o n o u r g e o m e t r i c a l f e a t u r e s . M o r e o v e r t h e y c a n s o l v e t h e l a t e r a l ' b l i n d n e s s ' c a u s e d b y t h e i n i t i a l c o n f u s i o n r e l a t e d t o d e n s e w e l l d a t a a n d b y t h e l a c k o f s e i s m i c d a t a .
A C K N O W L E D G E M E N T S
T h e w e l l l o g s a n d c o r e s a m p l e s w e r e p r o v i d e d b y M O L P i c . T h e a c a d e m i c l i c e n s e o f R o x a r I r a p R M S w a s p r o v i d e d b y E m e r s o n .
R E F E R E N C E S
G R I N G A R T E N , E . & D E U T S C H , C . V . ( 2 0 0 1 ) : T e a c h e r ' s a i d e - V a r i o g r a m i n t e r p r e t a t i o n a n d m o d e l l i n g , M a t h e m a t i c a l G e o l o g y , V o l . 3 3 , N o . 4 , 5 0 7 - 5 3 4 . P Y R C Z , M . J . & D E U T S C H , C . V . ( 2 0 1 4 ) : G e o s t a t i s t i c a l r e s e r v o i r m o d e l i n g - 2 n d e d i t i o n . O x f o r d U n i v e r s i t y P r i n t , U n i v e r s i t y o f O x f o r d , 4 4 8 p .
R I C H A R D S , M . & B O W M A N M . , ( 1 9 9 8 ) : S u b m a r i n e f a n s a n d r e l a t e d d e p o s i t i o n a l s y s t e m s I I : v a r i a b i l i t y i n r e s e r v o i r a r c h i t e c t u r e a n d w i r e l i n e l o g c h a r a c t e r , M a r i n e a n d P e t r o l e u m G e o l o g y , 1 5 , 8 2 1 - 8 3 9 .
