THE APPLICATION OF A NEW THERMAL MATURITY INDICATOR, THE PALYNOMORPH DARKNESS INDEX (PDI)

A new thermal maturity indicator, the Palynomorph Darkness Index (PDI) can be used to estimate the thermal maturity of rocks deficient in vitrinite, such as Lower Palaeozoics and some marine black shales. PDI integrates the measured intensities of red, green and blue light transmitted through palynomorphs using a digital camera mounted on a standard palynological microscope. Results are presented describing the calibration of PDI against vitrinite reflectance (VR) based on two Carboniferous sections that have very different thermal histories:

Amoco exploration well 33/22-1 was drilled in the Kish Bank Basin offshore Dublin, on the east coast of Ireland. This well penetrated a substantial Pennsylvanian section comprising sandstones, mudrocks and coals. VR results from this section suggest ‘normal’ maturation caused solely by burial with a high palaeogeothermal gradient ca. 115 ^oC/km.

In a coastal section near Ballycastle, Northern Ireland, late Mississippian sandstones, mudrocks and coals have been intruded by a major Tertiary dolerite dyke, the ‘North Star Dyke’. The maturity of mudrocks in the aureole of the dyke decreases from ca. 4 % Rr at the contact of the dyke to ca. 1.5 % Rr at the outer margin of the aureole. Here, the background maturity of the Carboniferous rocks is low and the increase in maturity within the aureole is caused by rapid heating associated with the intrusion.

Mean random vitrinite reflectance (R_r) was measured from samples in both sections using standard techniques. PDI was determined for unoxidised palynological preparations of mudrock samples based on the common Carboniferous miospore, Lycospora spp.

The results show that the functional range of PDI_Lycospora is from at least 0.9 to 2.5 % R_r (i.e. the oil window). There is excellent correlation between PDI and VR for normal maturation by burial and also for the samples within the aureole. However, an offset exists between the two sets of results which may provide useful information on heating rates.