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It is obvious that reservoir rocks that deposited in steep-slope at non-marine
basin margin in incredibly complex mixtures of physical, biological and chemical
processes, which have operated highly variable tectonic and climatic settings at the
lacustrine environment. The variations possible in such processes and settings make
interpretation of ancient reservoir extremely difficult, especially when they are
encountered by only a few boreholes in the subsurface,so the correct determination of
the character and origin of an ancient reservoir rocks, especially in the subsurface,
remains as one of the most difficult of tasks for the petro-geologist.
Because of this difficulty in dealing with subsurface reservoir description and in
order to provide a more accurate reservoir geological model for reservoir performance
prediction, it is necessary to adapt the method of multidisciplinary association and
static and dynamic information combination,to build an accurate interpretation model
of water-flooding reservoir operating parameters, a textural model and a heterogeneity
model before secondary or enhanced oil recovery. Therefore, set the Shan14-30 fault
block of Shanjiasi Oilfield as an example for the Steep Slope on the Margin in the
Faulted-Downward Basin, so based on synthetically analyses to the sedimentation,
sedimentary facies, diagenesis, heterogeneity and other geological factors, this paper
provides concretely dissecting and describing reservoir method.
Log data has been used to interpret porosity and permeability for different kinds
of reservoir rocks, this method is proved to be practical and advantageous by applying
the geological method. The analysis results of the paper are being applied to predict
the distribution of reservoir sandbodies. Moreover, the resent research has not only
made successful prediction for oil resources, sandbodies and so on in Shanjiasi Sag,
but also found a great useful tool in making an accurate description for the margin on
the steep slope in faulted-downward basin.
According to the regional unconformity and tectonic regime, basin fill
sequences can be divided into five tectonic sequences, each of which has the
individual properties of depositional association and the tectonic regime. The focus of
this paper is on the Fourth Member of the Shahejie Formation in Shanjiasi Oilfield.
Depositional systems in Shajiasi Sag are mainly fan-delta system, distributed on
the margin of the depositional basin. Based on the analysis of Fan-delta of Fourth
Member of Shahejie Formation in the Block Shan14-30, the formation background of
fan-delta discussed from the viewpoint of sedimentology. The facies marks and
sequence of fan-delta are summarized from a few aspects such as lithology, texture,
sedimentary structure, paleontology and distribution. In this paper the sub-facies and
the microfacies of fan-delta have been divided, the sedimentary models of fan delta
have been established, and the sandbody type of fan-delta is studied as well. It is
considered that the sedimentary characters are typical fan-deltas, the main
sedimentary features are formed by both tractional current and terbilital current, so
turbidity was well developed in Shanjiasi fan-delta. Because fan-delta is much near to
the sediment source than delta, so the transportation distance of sediment is very short
and the grain size is coarse, and the maturity of composition and texture is low. The
large-scale block bedding, parallel bedding and cross bedding were well developed.
Braided channel, interbraided channel fine-graded deposits, subwater distributary
channel and mouth bar were well developed. The vertical sequence is fining-upward
sequence, and the reservoir character is obviously controlled by sandbody type.
Three types of fan-delta sequences have been identified in Shan14-30 fault block,
the sequences suggest the evolution process of shallow-deep water envoirenment. The
north marginal contemporaneous fault controlled the sediment supply. Reservoirs in
this area are mainly various sand bodies in fan-delta front, among which river mouth
bars and distributary channels are more favourable.
Based on the macroscopic and the microscopic analyses of sandstone reservoir
rocks properties of Fourth Member of Shahejie Formation, the reservoir rocks are
characterized by big thickness, extensive distfibution The reservoir rocks contain
mainly moderate to micropores with relatively low permeability. It is obvious that the
heterogeneity of Fourth Member of Shahejie Formation varies greatly. The
heterogeneity of porosity and permeability of reservoirs are closely related to the
architectures of depositional systems, and can be studied by scale. On the scale of
depositional system, different depositional systems are different in porosity and
permeability. Porosity and permeability of different genetic facies in the same
depositional system may have great differences. This depends mainly on the paleo
current energy and environment in which the genetic facies were formed. On the scale
genetic facies, the porosity and permeability of the same fluid-flow unit and between
different fluid-flow units are on the whole different. This is related to lithology of
reservoirs constituting the fluid-flow units. The superimposition of non-homogeneous
diagenesis could decrease greatly the porosity and permeability of fluid-flow units. On
the scale of core, porous media show anisotrpic characteristics.
On the basis of the synthetically analysis of what talked above, some new
understandings about the reservoir in the steep slope have been gained as follows:
1.The steep slope in the margin of the continental fault basin is the favorable
area of the development of fan-delta. The fan-delta braided channel and subwater
distributary channels generally constitute the principal part of the reservoir sand body.
That the steep slope fan sand-conglomerate rock body is the main reservoir sand body
of lithological oil pool has been proved further by the research case of Shanjiasi fault
block The very changeable characteristic makes for the formation of lithological oil
pool. The steep slope fan-delta oil pool is one of the familiar types of reservoir of the
Tertiary fault basin in east China. Although the porosity and permeability of the steep
slope fan sand body is not high, the fan-delta front and profan-delta are individually
enclosed by shallow lake facies and relative deep lake facies deposition with the dark
mudstone, shale, mudstone with calcium and marl being all good source rocks which
is in favor of the upward migrating of hydrocarbon stored in the below source rock
and the laterally or downward migrating of hydrocarbon stored in the over source rock.
Accordingly, the fan-delta in the steep slope of the margin of the continental fault
basin is the favorable oil storage structure. The conclusion has been recently proved
by the practice of fine reservoir description done by the author in the central area and
south area in Jingan oil field in Changqing petroleum administrative bureau, Wei81
fault block in Weicheng oil field in Zhongyuan petroleum administrative bureau and
Cai36 fault block in Dongxin oil field in Shengli petroleum administrahve bureau.
2. There are three subfacies and eight microfacies that has been plotted out
through the analyzing of lithology, grain size, sedimentary structure, paleontology,
sedimentary cycle and electric log resources of the reservoir in study area in fan-delta
of the Forth Member of the Shahejie Formation in Shanjiasi Oil Field,i.e. fan-delta
plain subfacies which includes braided distributary channel and interchannel fine
particle sedimentation,fan-delta front subfacies which includes subsurface
distributary channel mouth bar, subsurface distributary channel and marginal sheet
sand microfacies and profan-delta subfacies which includes profan-delta silt,shale
sedimentation, storm turbidity and turbidity.
3. The depositional system of fan-delta in steep slope has different spatial
combination and configuration type along with the different stage of the initial slow
tiffing, early rapid rifting and terminal great rising of lake level of continental fault
basin: (1) The combination of fan-delta front, shallow lacustrine and profan-delta
(gravity flow) is shown at initial rifting stage. The combination which reflects the
evenness of the submarin landform is shown by the stability of each facies tract from
basin margin to deep lake area. The combination that fan-delta front and shallow
lacustrine comes forth alternately with deep lacustrine and gravity flow deposition is
familiar at the lower part of the Forth member of the Shahejie Formation; (2) The
combination of fan-delta plain, fan-delta front, profan-delta and deep lacusthne is
shown at early rapid falling stage when the marginal fault movement makes the
elevation difference between the denudation area and the deposition area increase
rapidly and clastic material supply abotmdant. The alluvial fan and fan-delta platn are
developed at the north margin of Shanjiasi Depression and transited to fan-delta front
southward, and then changed to profan-delta and deep lacustrine facies with gravity
flow deposition. This kind of sedimentary combination is developed at the upper
submember of the Forth member of the Shahejie Formation in Shanjiasi Depression;
(3)The combination of fan-delta front and profan-delta (deep lacustrine) is shown at
terminal rapid falling stage when the lake level rose greatly. This kind of combination
was developed at the upper part of the Forth member of the Shahejie Formation in
Shanjiasi Depression. The elevation difference between sedimention area and source
area increased further. The relative lake level rose rapidly. The supply of coarse
clastic material increased. There were abundant sediments in deep lake. Accordingly,
both fan-delta plain and profan-delta were not developed The whole sedimentary
combination is characterised by alternate bed of different thickness of the thick fan
delta front subfacies and the thin deep lacustrine facies with turbidi ty depoosition
4. The reservoir diagenesis in study area mainly includes compaction, pressure
solution, quartz secondary enlargement, cementation, the conversion from clay
mineral to illite, the corrosion of unstable minerals etc. The diagenesis rule can be
explained by the forming mechanism of inorganic and organic diagenesis. The quartz
secondary enlargement accounts for the leading station when pH less than 7. The
carbonate mineral is unstable when pH of the reservoir water less than 7, which
causes the corroding of the early carbonate cement and the forming of effective pore
that is in favor of the accumulating of petroleum. The carbonate mineral calcite and
ferroan calcite will be separated out and fill the interpartical pore and intrapartical
pore while the quartz and clay minerals become unstable with the secondary
enlargement of quartz being soluted or replaced by calcite when pH of the solution
greater than 7. The uneven diagenesis can severely decrease the whole level of
porosity and permeability of reservoir such as the uneven cementation ofcarbonate
rock fills primary pore and jams pore throat which causes the declining of the
reservoir physical property of sand body.
5. The heterogeneity of the Forth Member reservoir of the Shahejie Formation in
Shanl4-30 fault block is serious. The statistic result of physical property of each kind
of sand body is highly different The permeability of distributary channel sand body is
the best and debauch bar and frontal sheet sand body are in the next place and
interchannei fine particle sedimentary sand body is the worst; The subsurface
distributary channel sand body is the best and debauch bar is better by porosity. This
is consistent with the research result of core.
6. The sedimentary settings have obvious effect on reservoir heterogeneity. That
the facies change of fan-delta reservoir is quick is an important reason, which causes
the plane heterogeneity of steep slope deposition The fan-delta deposition has many
kind of microfacies type and the distributing range of each kind of microfacies is
relatively limited, the thickness of the sand body changes quickly laterally and thins
out and fingers out rapidly toward both sides. The difference of water body energy,
current velocity and load capacity of each facies tract causes the difference of rock in
mineralogy and texture.
7.The domination of plane heterogeneity to the distribution of residual oil and the
production op oil well is mainly affected by the connectivity of sand body and the difference of
permeability, for example, the sand body of the floodland and natural levee lied in both sides of
the braided channel is generally relatively thin and the mud content is relatively high. Accordingly,
the permeability is relatively low and the water drive effect is not good, the water flooded degree
is weak. So the abundance zone of residual oil are formed m such these areas. The interstratal
heterogeneity generally makes producing degree of each layer different. The produced degree is
high wherever the primary reservoir physical property is good. The produced degree is low
wherever the primary reservoir physical property is bad. So, it is easy to form the abumdance zone
of residual oil in areas talked above. The intrastatal heterogeneity mainly shows itself by
dominating the residual oil on the scale of microfacies, for example, the braided fluvial sand body
and subsurface distributary channel sand body takes on a characteristic that the lithology is coarse
at lower part and fine at upper part, the permeability is high at middle and lower part and low at
upper part.