Skip to main content

DIAGENESIS AND POROUS SYSTEM IN DANISH ZECHSTEIN CARBONATE RESERVOIRS.

Objective

THE AIM OF THIS PROJECT IS TO IMPROVE THE PRECISION IN THE CHARACTERIZATION OF GOOD-BAD HYDROCARBON RESERVOIR ZONES IN THE ZECHSTEIN CARBONATE ROCKS.
Selection of the basic parameters for seismic modelling is a critical step if relevant conclusions are to be drawn. Factors related to facies geometry, porosity development and velocity response have been investigated in Danish Zechstein (Upper Permian) carbonate reservoir rocks. The study concentrated on a single unit in the Danish Zechstein, the calcium carbonate which represents a potential reservoir interval. The results and principles developed during this work can be extrapolated to other carbonate reservoirs associated with hypersaline platform margins.
Flow properties within carbonate rocks are largely determined by the diagenetic evolution of the reservoir. The investigation was subdivided in 2 major fields of study: production study and velocity study. Considerable overlap exists between these 2 aspects. The reservoir geometry of Zechstein carbonate reservoirs is complex, in view of their close association with evaporite deposits. The platform margin oolites deposits appear to represent the best candidates for potential reservoirs.

The ultimate effect of diagenesis on reservoir quality depends on the delicate balance between dissolution processes and the introduction of late diagenetic anhydrite. Zechstein reservoirs have a high risk of anhydrite infection and it seems that only thick carbonate bodies could escape plugging of the porosity.

The common mouldic porosity developed as isolated vuggy porosity has an impact on production characteristics and also on the log interpretation of the sequences. Vuggy porosity lowers the hydrocarbon recovery and affects other parameters such as velocity and acoustic impedance. The comparison of laboratory measured velocities and sonic log data indicated that a velocity porosity equation should be developed for each specific region. A new modified equation to be used for Zechstein dolomites is proposed. In this Zechstein case, the 2-component lithology and the particular pore geometry necessitated modification of existing equations. This problem is analogous to that encountered in acoustic impedance interpretation from seismic data, where conversion to porosities and lithologies relies on an equation that is specific to that region.
THE SEISMIC MODELLING OF THE ZECHSTEIN RESERVOIRS REQUIRES MORE PRECISE INPUT DATA, DUE TO THE SMALL THICKNESS OF THE POROUS-PERMEABLE ZONES (LESS THAN 40 M AS AN AVERAGE) WHICH IS FREQUENTLY BELOW THE SIZE OF THE DIRECT SEISMIC RESOLUTION. MOREOVER, THE SOUND VELOCITY CONTRAST WITH RESPECT TO THE ANHYDRITE STRATE, WHICH INTERBED THE CARBONATES, IS LOW. MOST OF THE POROSITY OF THE INVOLVED ROCKS IS OF DIAGENETIC ORIGIN, HENCE THE FLOW PATTERNS IN THESE RESERVOIRS ARE CLOSELY LINKED TO THE SAME FACTORS THAT EFFECT SEISMIC RECOGNITIONS.
AFTER AN UPDATING OF LITERATURE DATA, A MATERIAL SURVEY (DRILLED CORES AND CORRESPONDING WIRELINE LOGS FROM NINE WELLS ARE AVAILABLE) AND SAMPLE SELECTION, A VELOCITY STUDY WILL BE UNDERTAKEN.
THIS WILL INCLUDE:
A) INTERVAL TRANSIT TIME LOG ANALYSIS TO CORRELATE, AFTER CAREFUL CORRECTION OF THE LOGS, THE CORED SEQUENCES TO THE UNCORED ONES AND TO INTERPRET BY MEANS OF COMPARISON, INTERVALS WHICH ARE NOT LOGGED;
B) ADJUSTMENT OF WIRELINE SIGNALS TO THE ACTUAL ROCK COMPONENTS AS CHEMICALLY DETERMINED ON CORE MATERIALS;
C) LAB ANALYSIS OF SOUND WAVE PROPAGATION AND POROSITY AT VARIOUS PRESSURES AND FOR DIFFERENT SATURATING FLUIDS.
A PARALLEL PRODUCTION STUDY WILL BE PERFORMED, INCLUDING: A) A PETROGRAPHIC CHARACTERIZATION OF THE ROCK FABRIC AIMED AT THE EXPLANATION OF THE VARIATION IN BOTH THE PRODUCTIBILITY OF THE PORE FLUID AND OF THE SOUND WAVE PROPAGATION. THIS WILL CONSIDER THE IMPORTANT TOPICS OF THE DIAGENETIC MODIFICATION OF THE ORIGINAL ROCK AND PORE TEXTURE, GRAIN SHAPE, CRYSTALLOGRAPHIC ORIENTATION AND TYPES OF INTERGROWTH OR POINT CONTACT;
B) IMAGE ANALYSIS OF PORE CASTS BY LAB PRODUCTION OF THEM AND ADJUSTMENT OF ANALYTICAL ROUTINES OF THE IMAGE ANALYSER;
C) LAB ANALYSIS OF TWO-PHASE FLOW AND SOUND WAVE PROPAGATION TIME IN VARIOUS DIRECTIONS;
D) NMR SCANNING ON SELECTED SAMPLES.
AN IMPROVED PREDICTION THROUGH INDIRECT SEISMIC INTERPRETATION OF RESERVOIR LITHOLOGY AGAINST SEALING ANHYDRITE AND ROCK SALT,AND DISTINCTION BETWEEN TYPES OF PRODUCEABLE ZONES WILL ENSUE. A SEISMIC MODEL TEST BASED ON THE DATA FROM THE AFOREMENTIONED STUDIES WILL BE PERFORMED.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Danmarks Geologiske Undersøgelse
Address

2400 København Nv
Denmark