The American Association of Petroleum Geologists Bulletin. V. 77. No. ... tion is in the Iraq geologic lexicon (Dunnington, 1959), where it ..... Scale bar = 1 mm.
The American Association of Petroleum Geologists Bulletin V. 77. No. 11 (November 1993). P. 1971-1988, 11 Figs.. 3 Tables
Stratigraphic Sequence, Microfacies, and Petroleum Prospects of the Yamama Formation, Lower Cretaceous, Southern Iraq1 FadhilN. Sadooni2
ABSTRACT
The Lower Cretaceous Yamama Formation basin
was deposited over two tectonic regimes, the north eastern flank of the stable Arabian platform and the unstable zone of the Mesopotamian foredeep. This sit uation and the probable syntectonic deposition of the formation over growing structures created complex carbonate lithologies. From this tectonic configuration and the facies distribution, it seems that the Yamama
Formation was deposited in a setting that changed from an inner to an outer ramp. Along the hinge line
separating those two tectonic units, oolite shoals developed. The crestal areas of the growing structures were occupied by stromatoporoid-sponge-coral reefs and oolitic facies. The areas between reefs were the sites of carbonate mud accumulation.
For the purposes of regional correlation and reserve estimation, the Yamama Formation is divid ed into five lithologic units: three reservoir units, designated from top as YR-A, YR-B, and YR-C, sepa
rated by two permeability barrier units, YB-1 and YB2. These reservoir units are thought to be at least
partially isolated from each other. The best oil prospects are within the oolite shoals and the patch reef buildups in the crestal parts of the structures.
formation. Although a brief description of the forma tion is in the Iraq geologic lexicon (Dunnington, 1959), where it was described under a combined entry with the Sulaiy Formation. The nomenclature used here is borrowed from the Saudi stratigraphic system. The Yamama Formation belongs to the late Berri-
asian-Aptian cycle. This cycle is represented from shore to deep basin by the Zubair, Ratawi, Garagu, Yamama, Shuiaba, Sarmord, and Lower Balambo For mations (Buday,1980). The Yamama Formation is made up mainly of limestone, but some dolomitic limestone and shale have been reported. In the southwestern part of the Yamama basin, some wells contain anhydrite within the Yamama sections. The formation is underlain conformably by the Sulaiy Formation. This formation is an uppermost
Jurassic limestone made up of mud-supported argilla ceous limestone with calcispheres and small benthonic foraminifera.
The Yamama grades upward into the Ratawi For mation, which is a heterogeneous suite of limestone, shale, siltstone, and sandstone. The Ratawi Forma
tion, in turn, grades locally into the Zubair sand stone-shale formation. The Ratawi is the cap rock of the Yamama reservoir. Where the Ratawi thins out
or disappears, the Zubair Formation directly overlies the Yamama, as is the case in some wells on the western side of the basin (Figure 1).
The Sulaiy, Yamama, Ratawi, and Zubair forma tions represent a regressive carbonate cycle termi nated by the clastic invasion of the Zubair fluvial-
INTRODUCTION
The Yamama Formation is the main Lower Creta
ceous carbonate reservoir in southern Iraq. The pre
deltaic facies. The Yamama Formation comprises the neritic lithofacies of the cycle.
sent study is the first detailed published study on this ©Copyright 1993. The American Association of Petroleum Geologists. All
PREVIOUS WORK
rights reserved.
1 Manuscript received, March 3, 1992; revised manuscript received, January 15.1993; final acceptance, February 2,1993. 2Department of Earth and Environmental Sciences, Yarmouk University, Irbid, Jordan.
This work has benefited from discussions with former colleagues at the
Iraqi Oil Exploration Company, including Ahmad Al-Siddiki, Adnan AlSamarrai, Sameer Saeed, Farid Iskendrian, Juniya Batani, and Khairiya Hassan. The author is indebted to the AAPG reviewers Ziad Beydoun,
James Peterson, J. F. Sarg, James Lee Wilson, and an anonymous reviewer for reading the original manuscript and suggesting valuable improvements. Nizar Abu Jaber read the final version of the paper.
The Yamama Formation was first described by
Steinke and Bramkamp (1952) in an abstract about the Mesozoic rocks of Saudi Arabia. They mentioned that the Yamama Formation represents the Thama-
ma Group, along with the Buaib and Sulaiy Forma tions. According to them, the Yamama Formation is made of Early Cretaceous fragmental limestone. Dunnington (1959) described the Yamama Forma-
1971
1972
SAUDI
Yamama Formation, Iraq
IRAQ
QATAR KUWAIT
ARABIA
EPOCH
SOUTH
WEST
NORTHERN OIL FIELDS-N&E KURDISTAN
i
CENOMANIAN
ALBIAN
APTIAN BARREMIAN HAUTERVIAN
VALANGINIAN
BERRIASIAN CHIA
GARA
TTTHONIAN
Figure 1—Stratigraphic relations of the Yamama Formation and its equivalents in Iraq and neighboring countries (Iraqi Oil Exploration Company archives). J= Jurassic.
tion and combined it with the Sulaiy Formation. He indicated that this formation, which he described
from a complimentary type section in the Ratawi-1 well and from the Burgan-113 well in Kuwait, con sisted of pelletal limestone that underlies the Ratawi Shale Formation in southern Iraq. According to Rabanit (in Dunnington, 1959), the section designat ed as the Yamama/Sulaiy Formation in the Ratawi-1 well is composed mainly of limestone. Its lithologies are given in Table 1. In an unpublished report, Chatton and Hart
Table 1. Main Lithologic Components of the Yamama Formation in Well Ratawi-1*
Lithology
Thickness
12 79
23 11
3313 and 3566 m. The main lithologic units are tran scribed in Table 2. In the remarks attached to the
definition, these authors mentioned that this amend
ment was necessary to emphasize the lithologic dif ferences between the Ratawi and Yamama forma
tions. Both formations are neritic deposits, but, according to Chatton and Hart, the term "Yamama"
will be used to embody the carbonate components, whereas, the Ratawi refers to the clastic facies that include sandstone, siltstone, shale, and marl. Al-Siddiki (1977) redescribed the Yamama Forma
tion adhering to the above definitions. The top of the Yamama was the first appearance in wells of the clean limestone unit occurring below the shale and the argillaceous limestone of the Ratawi Formation. The
lower contact was placed at the first appearance
(m)
190
(1962) amended the definition of the Ratawi Forma tion from the Ratawi-1 well between the depths of
Detrital limestone, fine-grained, interbedded with oolitic and pseudoolitic limestone. The limestone may contain Pseudocyclammina off. lituus (Yokoyama), Trocholina spp., and Spirocyclina spp. The presence of Terebratula sp. and Globularia sp. was reported by Hudson in an unpublished report cited by Dunnington (1959) in the lower part of this section. Brownish, spicular, detrital limestone. Brownish, detrital limestone, shaly in parts and rich with organic matter. Pseudocyclammina spp., Trocholina sp., and small miliolids may be present. Recrystallized limestone. Dense, dark-brown limestone with streaks of
arenaceous shale showing current beddings with some calcispheres. "After Dunnington (1959).
downward of argillaceous limestone characteristic of
Table 2. Lithology of the Ratawi Formation in Well Ratawi-1*
Lithology
Thickness
(m)
6
Dense, organic, detrital limestone.
21
Shale with some limestone streaks.
2
Marl and some greenish gray shale with thin limestone at base.
34
Black shale with two sandstone units.
162
Greenish gray calcareous shale alternating with dense, organic detrital limestone changing into marly limestone in some locations.
*AtterChatton and Hart (1962).
Sadooni
the Sulaiy Formation. The formation was divided into
1973
STRATIGRAPHIC SEQUENCE
three main lithofacies. The first from the bottom is the
porous, permeable carbonate composed of reworked carbonate detritus; the second lithofacies is dense, fine-grained limestone; and the third lithofacies is oolitic limestone with high porosity and permeability. Ditmar (1971) indicated that the rocks assigned to the Ratawi and Yamama formations contain, in cer tain areas, fossils belonging to the Tithonian-late Berriasian periods. In other areas (mainly in north ern parts of Iraq), the Yamama sediments are of
Valanginian and Barremian age. According to Buday
The main controversy associated with the stratig raphy of the Yamama Formation is the delineation of
the contact between this formation and the overly ing Ratawi Formation.
The West Qurna-14 well was chosen as a refer ence well to fix this contact because it had been well cored in the transition zone between the two formations. Based on information from the core
description and the thin section analysis done on the previously determined contact between the Ratawi
(1980), what is described as a Yamama Formation in
the western part of Iraq (Ghlaisan, Samawa, and Kifl wells) is Berriasian in age and should be assigned to the Sulaiy Formation, whereas the sediments belong ing to the Yamama are found mainly in the eastern part of the unstable shelf in southern Iraq. In this
*.
TURKEY
work, the term "Yamama Formation" will be used as
a lithostratigraphic rather than a chronostratigraphic term because the formation may be dichronous. The geology of the Ratawi, Yamama, and Sulaiy formations was studied by Al-Siddiki (1978a, b) in southern Iraq. His studies were prepared for reser voir evaluation purposes. Consequently, the studies were concerned mainly with the vertical subdivi sions of these formations. Accordingly, Al-Siddiki divided the Ratawi into three reservoir units (from
the top) RA, RB, and RC. The Yamama also was divided into three units assigned (from the top downward) as YA, YB, and YC. These units, more over, can be subdivided into subunits such as YA-1,
YA-2, etc. The Sulaiy Formation, in turn, was divided (from the top) into five units: SA, SB, SC, SD, and SE. Samples in the form of cores, cuttings, and well logs were chosen from selected oil wells from all over the southern part of Iraq. The studied wells are shown in Figure 2B.
SAUDI ARABIA
Figure 2—(A) Tectonic map of Iraq (After Buday and Jassim, 1983). (B) Location map of the stud ied wells and the correla tion sections.
Yamama Formation, Iraq
1974
(A) KI-l SP
SONIC
©
OOLTTH
•
PELOID
O
LU-12 SP
WQ-1S
RT-3
SONIC
SONIC
SP
SONIC
MJ-3 SP
A'
SONIC
CORAL & STROMATOPOROID
(J
FORAMINIFERA
*•
ALGAE
d
BIOCLAST
n=l
ARGILLACEOUS
LtJ
LIMESTONE
ISil
SHALE
NO HORIZONTAL SCALE
Figure 3—(A) AA1. Regional correlation of the Yamama Formation in the Khider Alma-1 (KI-1), Luhais-12 (LU-12),
Ratawi-3 (RT-3),West Qurna-15(WQ-15), and Majnoon-3 (W-3) wells. See Figure 2 for location. (B) BB'. Regional cor relation of the Yamama Formation in the Tuba-2 (TU-2), Rumaila South-72 (RU-72), Dibdiba-1 (DI-1), Rachi-2 (RC-2), and Luhais-12 (LU-12) wells. See Figure 2 for location. (C) CC. Regional correlation of the Yamama Formation in the Ghlaisan-1 (GH-1), Samawa-1 (SM-1), Nasiryia-1 (NS-1) and Halfaiya-2 (HF-2) wells. See Figure 2 for location.
and Yamama formations that the so-called unit YA-1
(Al-Siddiki, 1977) consists of silty sandstone and siltstone. This is contrary to the criteria used by Chat ton and Hart (1962), for differentiating between the two formations. Accordingly, the contact was slight ly lowered below this unit and the top of the Yama
distinctive log pattern of the Sulaiy Formation on the gamma-ray and spontaneous potential (SP) logs. A fair consistency was observed in most of the studied wells and the differences do not exceed a few meters. The
Iraqi Oil Exploration Company decided to keep the
carbonate. The new contact was then traced in all
vertical stratigraphic subdivisions of the Yamama For mation as suggested by Al-Siddiki (1978a) because they are already in use on a wide scale. The main problem associated with this scheme of classification
the studied wells. This resulted, at least for some wells, in changes of 30 m in the formation thickness.
is that these units are not correlatable over large areas. Previous regional correlation attempts resulted in mis-
Some of the correlated wells are shown in Figure 3. Establishing the lower contact with the underlying
correlation even within the same oil field.
ma Formation consequently has been delineated on
the first downward appearance of a relatively pure
Sulaiy Formation was less problematic because of the
Miscorrelation was due to the wide, but gradual, lithologic differences exhibited by the individual
Sadooni
1975
(B) B
TTJ-2 si>
SONIC
RU-72 SP
SONIC
SP
LU-12
RC-2
DI-1 SONIC
SP
SONIC
SP
B'
SONIC
NO HORIZONTAL SCALE
Figure 3—Continued.
units, that is, wackestone-bearing algae and peloids would change into wackestone-bearing algae and foraminifera in a neighboring well, and then into wackestone-bearing foraminifera and echinoderms in a third well. In these wells, the first facies resem bles the second facies by 50%, but does not imitate
the third. These units were subjected to different
diagenetic processes or to variable degrees of the same process, which leads to local changes in their petrophysical properties and their log signatures. In addition, prior topographic surface, tectonism, and
widely distributed and display very little change in their lithology. The first barrier unit is the YB-1 (Yama ma barrier one),which was correlated throughout the study area. The first reservoir unit YR-A (Yamama reservoir A) was confined between the top of the for mation and the top of barrier unit YB-1. This has made it possible to correlate the unit over the study area despite any changes in its petrophysical properties. A similar correlation is possible using barrier unit YB-2, and so the reservoir unit YR-B was delineated
between YB-1 and YB-2. The isopach map of the total
different rates of subsidence may lead to consider
thickness of the Yamama Formation is shown in Fig
able variations in member thicknesses.
ure 4, which is the first map following those of Flouria (1976) and Al-Siddiki (1977) and is based on additional control points. The three maps have many
On the contrary, the thin, dense, argillaceous lime stone units that form the permeability barriers are
1976
Yamama Formation, Iraq
(C) C
GH-1 SP
SONIC
SP
SONIC
HF-2
NS-1
SM-1 SP
SONIC
SP
c
SONIC
NO HORIZONTAL SCALE
Figure 3—Continued.
similarities, but the new map shows a considerable
megasequence consists of the reservoir unit YR-A.
reduction in the thickness of the Yamama after
The separating unit between the two megase
redefining its upper contact. Clearly, the Rumaila South-72, Dibdiba-1, and to a lesser extent, the Jeris-
quences is the barrier unit YB-1.
han-1 and Tuba-2 wells show a considerable reduc
tion in thickness due to the thinning in the YR-C unit. The Yamama Formation, accordingly, is divided into two main megasequences. The lower megase
TECTONIC SETTING
quence includes the two reservoir units, YR-C and
ing the final configuration of the Yamama basin, and
YR-B, with the barrier unit YB-2, and the upper
Tectonism has played an important role in shap the Yamama basin extends over two different tec-
Sadooni
1977
tonic zones. The western part of the basin is situated within the stable shelf of the Arabian Platform
(specifically the Salman subzone of that belt), and the eastern part of the basin is located within the unstable shelf, the Mesopotamian foredeep (Figure 1). The hinge line between these two tectonic units is the northeastern slope of the Arabian platform that passes near the Khider Alma-1 well and extends
IRAN
toward the Kuwait borders. Al-Banna and Al-Rawi
(1991) noticed that the eastern region of Iraq is char acterized by a positive correlation between Bouger anomalies and elevations, whereas the western region is marked by a generally negative relation.
The hinge line between these two regions appears to follow the zero isostatic anomalies isoline.
The Mesopotamian foredeep has experienced active syntectonic sedimentation leading to the for mation of giant structures that were apparently simultaneously growing during deposition of the Yamama Formation. These structures probably were
% Jer§han-l-6>
induced by diapiric warping caused by the Infracam-
Khid^cAlma-r/
brian Hormuz Salt Series which is believed to under
*/
line parts of southern Iraq (Buday, 1980). Bolton (1958) suggested that the main folds in Iraq might be formed by the presence of salt layers of considerable thickness. Al-Naqib (1970) concluded that Sanam Mountain on the Iraqi-Kuwaiti borders was formed
LEGEND
A
Oil. WILL
•A- WELL WITH OIL SHOWS
as a result of salt intrusion. The presence of salt lay ers was confirmed by Ditmar (1971) from anomalous features on the gravity maps of southern Iraq.
According to Rona (1982) and Burchette and Wright (1992), rift basins are preferred sites for salt accumula tions. The Hormuz Salt Series deposition was followed by the Paleozoicclastic sediments belonging to the Saq,
KUWAIT .A. DRY WELL
100 Km coflrrovM valves ovmbtezs
Figure 4—Isopach map of the Yamama Formation in southern Iraq.
Khabour, and Gaa'ra formations. The Mesozoic carbon
the basin, the unit is made of algal limestone with benthonic foraminifera, corals, and stromatoporoids
ate province was established upon these elastics.
(Rumaila North-172, West Qurna-15, Zubair-42, and Nahr Umr-7 wells). The YR-A unit in the northern
part of the basin is composed of argillaceous lime LITHOLOGIC DESCRIPTION
The Yamama Formation is made of two perme
ability barrier units (YB-1 and YB-2, from the top down) and three reservoir units (YR-A, YR-B, and YR-C, from the top down). The following is a short summary of their lithologic characteristics.
Reservoir Unit YR-A
stone containing fine debris of red algae, and shell
fragments in the eastern part of the Yamama basin, (Halfaiya-2, Noor-1, and Kumait-1 wells). The unit shows two areas of maximum thickness. The first area is near Ratawi oil field and the second
area is near Majnoon oil field. The maximum thick ness of the unit appears to be associated either with the development of oolite shoals or with the patchy reefs at the tops of the largest structures. The lithologic character of the unit may change with the location of the well on the structure. In gen
This unit shows a wide variation in both lithology and thickness. In wells with high total Yamama thickness, this unit is thick also. Figure 5A is an
isopach map of this unit. Note that the unit attains its greatest thickness (100 m) at the Ratawi-3 well. The unit consists of limestone with oolites, pseu
eral, crestal wells have purer sections of the unit than flanking wells.
Barrier Unit YB-1
do-oolites, pellets, peloids, and bioclasts in the west
Unit YB-1 represents the main siliciclastic-dominat-
ern area of the basin (Samawa-l,Ghlaisan-l, Abu-
ed facies in the formation. This unit is made of shale,
Khima-1, and Ratawi-3 wells). In the central parts of
argillaceous mudstone, lime mudstone, and wacke-
1978
Yamama Formation, Iraq
IRAN
tachi 2* Di^iba-1^ \
^-—jo >*
'--^
_.._
( V
Khider AIma-l7
♦ /
KUWAIT
Figure 5—(A) Isopach mapof the reservoir unitYR-A. (B) Isopach mapof the barrier unitYB-1. (C) Isopach mapof the reservoir unitYR-B. (D) Isopach map of the barrier unitYB-2. (E) Isopach mapof the reservoir unitYR-C. Leg end for oil wells as in Figure 4.
stone, and is absent in some of the western wells. The
isopach map of the unit Figure 5B suggests that the unit represents a lowstand wedge in a basin-center position. On the shallower parts of the basin, the unit is represented by hardgrounds with petrified plant remains and oxidized mudstone. The origin of these fine clastic materials is situated far away from the shoreline and is not well understood. These elastics
probably represent outer ramp deposits and the ter rigenous mud may be of suspension origin. Reservoir Unit YR-B
This unit shows a wide variation in lithology. Moreover, it may contain oil, bitumen, or water. The
YR-B unit is composed of oolitic and algal bioclastic limestone in the Ratawi-3 and Luhais-12 wells. In
West Qurna oil field, YR-B is composed of limestone containing peloids, pseudo-oolites with stromato poroids, and corals. The unit changes facies into argillaceous mud-supported facies in the eastern and northeastern fringes of the basin, as in the Noor-1 and Kumait-1 wells where it is amalgamated with unit YR-C, as well in the southern part of the basin in wells such as Dibdiba-1 and Rumaila South-72. YR-B is difficult to differentiate from unit YR-C because
both units are made of argillaceous limestone with poor petrophysical properties. Figure 5C is an isopach map of this unit, which obviously shows that the greatest thickness of the formation can be
found in the Rumaila North, West Qurna, Zubair, Nahr Umr, Ratawi, Tuba, and Luhais wells. Minimum thickness is in the Khider Alma-1 well. The unit can
not be separated from the reservoir unit YR-C in the Halfaiya-2 well to the east.
Sadooni
N
©
© f
Kumait-1
Rafedam-lnuJau"a-2-$-
Noor-1
IRAN
\
1979
N
v
Kumait-1 Kumait-1
"s. N--
Rafedain->r^J^^N>-\ NooJ;1
^\\
HaIfaiy#a-2
\
/
IRAN
Khid?
^vf KUWAIT
KUWAIT
Abu Khima-1
Abu Khima-1
-'
•
/ /
.s
0
0
100 Km
100 Km
Figure 5—Continued.
Barrier Unit YB-2
This unit separates the two reservoir units YR-B and YR-C. YB-2 is of less consistency than YB-1 and
its argillaceous content is lower. Generally, the unit is formed of dense mudstone and wackestone and
attains its greatest thickness at the Zubair-42 well. Figure 5D is an isopach map of this unit. The maxi
remaining wells, YR-C is of varied lithology ranging from lime mudstone to algal-foraminiferal packstone with zones of oolitic and peloidal limestone. Its petro
physical properties become less distinctive with depth, changing into argillaceous limestone at the top of the SulaiyFormation. Figure 5E is an isopach map of this unit, which seems to have its greatest thickness between the Zubair-42 and Garaf-1 wells.
mum thickness of the unit can be found in the south
eastern part of the basin, south of Zubair oil field. MICROFACBES
Reservoir Unit YR-C
This unit represents a transitional zone between the Yamama and the Sulaiy formations. The petrophysical properties of the unit are of poorer quality as com pared to YR-B and YR-C. With the present level of available information, one cannot judge whether this unit is totally isolated from reservoir unit YR-B because this depends on the effectiveness of unit YB-2 as a bar rier. YB-2 is absent in the eastern part of the basin, where units YR-B and YR-C grade into argillaceous limestone. YR-Cgenerally consists of oolitic limestone in the Khider Alma-1 and Abu Khima-1 wells. In the
The wide variation in the lithology and facies of the Yamama Formation may be due to the following. (1) The Cretaceous represents, according to Murris (1980), the beginning of the entrance of the Arabian Gulf area, including south Iraq, into the subtropics. This change led to an extensive differentiation in the abundance and nature of both sediments and biota. (2)
The Yamama was deposited on an irregular paleotopo-
graphic surface. There are some indications of tectonically induced topography that probably predate Yama
ma deposition. In fact, some lithologicvariation of the formation occurs within the same structure because of
variation in the symmetry, slope, and probable local
1980
Yamama Formation, Iraq
N
% Kumait-1
iila-2 a
v
Noor-1
\
!as" . , vHalfai£a-2
> IRAN
/
stone, shale, and marl. Generally, the clayey contents are relatively high. This facies is the main component of the barrier units YB-1 and YB-2. In some wells, the
facies changes into calcareous shale containing quartz grains. The same facies was found also within the reservoir units YR-B and YR-C in the Dibdiba-1 and Rumaila South-72 wells and in wells drilled in the
southern parts of the Zubair oil field. These sediments probably were formed in an outer ramp setting. The term "lime-mudstone facies" is used here in
the manner suggested by Dunham (1962) to indicate rocks made up of pure lime mud (Figure 6A). Approximately 50 m of this facies is in the West Qurna-14 well. Sadooni (1979) described this facies as being made of well-sorted micrite grains with good matrix porosity and chalky texture (Figure 6B). Previous workers (e.g., Dunnington 1959; Al-Sid diki 1977,1978a) mentioned that this facies contains
probable sponge spicules. Sponges in lagoons and sheltered areas may baffle and trap lime mud form ing mud mounds. Khider
KUWAIT
Abu Khima-1
#
/
/ 0 i
100 Km
5 i
Figure 5—Continued.
faulting of that structure, which may be the main con trolling factor. (3) The basin is divided between two different tectonic units. (4) General changes in the sea level during deposition and the position of the Yama ma basin relative to the wind direction helped vary the lithology and facies. These factors led, collectively, to the development of complex facies patterns, both ver tically and horizontally, in ways that make it impossi ble, at times, to correlate between two neighboring wells. The main microfacies of the formation are mud
stone, algal-bearing wackestone-packstone, large foraminifera wackestone and packstone, oolitic packstone-grainstone, peloidal packstone-grainstone, lithoclastic wackestone-packstone, and stromatoporoidsponge-coral boundstone.
Mudstone Microfacies
The mudstone microfacies is divided into two main
Algal-Bearing Wackestone and Packstone This facies is equivalent to the algal debris facies described by Elliott (1958) (Figure 6C) and may be divided according to algae types into two subfacies. The dasycladacean wackestone-packstone subfacies is mainly made of green algae from the Dasy cladacean group. Most of their skeletons were dis solved to be filled by sparry calcite cement. In most cases, algae are mixed with shell fragments (mainly pelecypods). This subfacies may also contain minor large benthonic foraminifera, such as Pseudocy clammina spp. and Everticyclammina spp. Those skeletal debris were scattered in slightly argillaceous matrix. Green algae are known to prefer normal marine waters, but may be found in isolated lagoons. Green algae favor tropical to subtropical areas and flourish in subtidal depths of 3-5 m and range to depths of 30 m. The water usually is of low energy and the algae live below wave base. The sea floor may be muddy or sandy. Generally, sheltered lagoons and protected reef flats are the most favor able areas for green algae production (Wray, 1977). The red algae wackestone-packstone consists of Permocalculus, the main red algae group preserved as fossils (Johnson and Kaska 1965). Most of the Yamama red algae belong to this group, which is characterized by perforated skeletons. Most of them are well preserved due to extensive filling by calcite cement. The most important red algae species are Permocalculus ampullacea, Permocalculus walnutenses, and Permocalculus irena.
submicrofacies: argillaceous mudstone and lime mud stone. The argillaceous mudstone typically is made of
(1958) in the Lower and Middle Cretaceous of the
dark, dense micrite with considerable amounts of silt-
Middle East and by Johnson and Kaska (1965) in
This facies has been described earlier by Elliott
Sadooni
1981
i
*** %
,-
