Offshore Carbonate Facies Characterization and Reservoir Quality of Miocene Rocks in the Southern Margin of South China Sea

Carbonate rocks are important hydrocarbon reservoirs around the globe and in Southeast Asia a key one,particularly,is the Central Luconia province.Understanding the internal characteristics,distribution,geometry and lateral extent of these rocks is essential for exploration and production success.Detailed work on Miocene carbonate reservoir facies,north of Bintulu,offshore Sarawak includes qualitative and quantitative analysis of photomicrographs and reservoir quality,considering especially microporosity.Stratigraphically,these carbonates are known as Cycles IV and V and are represented by eight major facies types(F-1 to F-8).They comprise:coated grain packstone(F-1)(av.Ф=3%,av.Kh=0.5 m D)(av=Average;?=total porosity,and Kh=permeability);massive coral lime grainstone(F-2)(av.Ф=14.7%,av.Kh=6 m D);oncolite lime grain-dominated packstone(F-3)(av.Ф=10%,av.Kh=4 m D);skeletal lime/dolo-packstone(F-4)(av.Ф=15%,av.Kh=4.6 m D);coral(platy)lime mud-dominated packstone(F-5)(av.Ф=4%,av.Kh=0.5 m D);coral(branching)lime-dominated pack-grainstone(F-6)(av.Ф=15%,av.Kh=1 m D);cross-bedded skeletal lime packstone(F-7)(av.Ф=20%,av.Kh=2 m D);and bioturbated carbonate mudstone/chalk(F-8)(av.Ф=8%,av.Kh=0.8 m D).Study of thin sections reveals that red algae,foraminifera,and corals are the dominant fossil components with a minor admixture of skeletal fragments of echinoderms,bivalves,bryozoans,and green algae.All parameters,e.g.,facies characterization,petrography,porosity–permeability value,and microporosity value were utilized to obtain a reliable reservoir quality.The microporosity value was quantified using digital image analysis software and is significant for recognition of good reservoir quality.Consideration of the presence of microporosity against the total porosity in the carbonate rocks has improved the correlation coefficient(R2)value,which has increased from 0.51 to 0.82.

Pore-throat structure characteristics and its impact on the porosity and permeability relationship of Carboniferous carbonate reservoirs in eastern edge of Pre-Caspian Basin

Carboniferous carbonate reservoirs at the eastern edge of the Pre-Caspian Basin have undergone complex sedimentation,diagenesis and tectonism processes,and developed various reservoir space types of pores,cavities and fractures with complicated combination patterns which create intricate pore-throats structure.The complex pore-throat structure leads to the complex porosity-permeability relationship,bringing great challenges for classification and evaluation of reservoirs and efficient development.Based on the comprehensive analysis on cores,thin sections,SEM,mercury intrusion,routine core analysis and various tests,this paper systematically investigated the features and main controlling factors of pore-throats structure and its impact on the porosity-permeability relationship of the four reservoir types which were pore-cavity-fracture,pore-cavity,pore-fracture and pore,and three progresses are made.(1)A set of classification and descriptive approach for pore-throat structure of Carboniferous carbonate reservoirs applied to the eastern edge of the Pre-Caspian Basin was established.Four types of pore-throat structures were developed which were wide multimodal mode,wide bimodal mode,centralized unimodal mode and asymmetry bimodal mode,respectively.The discriminant index of pore-throat structure was proposed,realizing the quantitative characterization of pore-throat structure types.(2)The microscopic heterogeneity of pore reservoir was the strongest and four types of pore-throat structures were all developed.The pore-fracture and pore-cavity-fracture reservoirs took the second place,and the microscopic heterogeneity of pore-cavity reservoir was the weakest.It was revealed that the main controlling factor of pore-throat structure was the combination patterns of reservoir space types formed by sedimentation,diagenesis and tectonism.(3)It was revealed that the development of various pore-throat structure types was the important factor affecting poroperm relationship of reservoirs.The calculation accuracy of permeability of reservoirs can be improved remarkably by subdividing the pore-throat structure types.This study deepens the understanding of pore-throat structure of complicated carbonate reservoirs,and is conducive to classification and evaluation,establishment of precise porosity-permeability relationship and highly efficient development of carbonate reservoirs.

Reservoirs properties of slump-type sub-lacustrine fans and their main control factors in first member of Paleogene Shahejie Formation in Binhai area, Bohai Bay Basin, China

High-yielding oil wells were recently found in the first member of Paleogene Shahejie Formation,the Binhai area of Qikou Sag,providing an example of medium-and deep-buried high-quality reservoirs in the central part of a faulted lacustrine basin.By using data of cores,cast thin sections,scanning electron microscope and physical property tests,the sedimentary facies,physical properties and main control factors of the high-quality reservoirs were analyzed.The reservoirs are identified as deposits of slump-type sub-lacustrine fans,which are marked by muddy fragments,slump deformation structure and Bouma sequences in sandstones.They present mostly medium porosity and low permeability,and slightly medium porosity and high permeability.They have primary intergranular pores,intergranular and intragranular dissolution pores in feldspar and detritus grains,and structural microcracks as storage space.The main factors controlling the high quality reservoirs are as follows:(1)Favorable sedimentary microfacies of main and proximal distributary gravity flow channels.The microfacies with coarse sediment were dominated by transportation and deposition of sandy debris flow,and the effect of deposition on reservoir properties decreases with the increase of depth.(2)Medium texture maturity.It is shown by medium-sorted sandstones that were formed by beach bar sediment collapsing and redepositing,and was good for the formation of the primary intergranular pores.(3)High content of intermediate-acid volcanic rock detritus.The reservoir sandstone has high content of detritus of various components,especially intermediate-acid volcanic rock detritus,which is good for the formation of dissolution pores.(4)Organic acid corrosion.It was attributed to hydrocarbon maturity during mesodiagenetic A substage.(5)Early-forming and long lasting overpressure.A large-scale overpressure compartment was caused by under-compaction and hydrocarbon generation pressurization related to thick deep-lacustrine mudstone,and is responsible for the preservation of abundant primary pores.(6)Regional transtensional tectonic action.It resulted in the structural microcracks.