Module 7: Basin Analysis

Well Logs Versus Seismic Logs

Well Logs:

• great vertical resolution

• delimit bounding surfaces

• establish lithology of penetrated

Seismic Logs:

• great lateral continuity and resolution

• define gross sediment geometry

What causes mudcake

Hydrostatic pressure of the mud column is usually greater than the pore pressure of the formations

This forces mud to filtrate into the permeable formations and a mud cake on the borehole wall

How does tool sensor relate to lithology?

1. Caliper sensor measures the size of the drilled borehole

2. electrical sensors: detect porewater amount, permeability, and ionic concentration of sediment and porewater

3. Gamma-ray sensors: measure the amount and type of radioactive elements (U, Th, K) in sediment

Spontaneous Potential (SP) Logs

• Measures electrical current in well

• Result of salinity differences between formation water and the borehole mud

• Separates bed boundaries of permeable sands and impermeable shales

Resistivity Logs

• Measures resistance of flow of electric current

• Is function of permeability, porosity and pore salinity in rock

• Frequently used to identify lithology that varies based on permeability

• High Resistivity= little porewater

• Low resistivity= more porewater

Gamma Ray Logs

• records natural radioactivity of formation

• shales have high gamma radioactive response

• gamma ray logs infer grain size

• gamma ray logs are more commonly used logs for sequence stratigraphic analysis

Acoustic Impedance

Seismic Wave Velocity x Density

Seismic sequence analysis

Allostratigraphy

Bounding discontinuities including erosion surfaces, marine flooding surfaces, tuffs, tempestite, and/or turbidite boundaries as time markers

Sequence Stratigraphy

higher level allostratigraphic model which interprets depositional origin of sedimentary strata as products of “relatively sea level change”

Sequence Stratigraphy – Base Level

Boundary Types

Disconformity: unconformity separating parallel strata where there has been significant erosion

Paraconformity: unconformity between parallel strata where there has been little erosion

Angular unconformity: unconformity where tilting and erosion of strata has been followed by deposition

Onlap

Onlap is baselap in which an initially sub-horizontal stratum laps out against an initially inclined surface

Both onlap and downlap are indicators of non-depositional breaks (hiatuses).

Downlap

Downlap is baselap in which an initially inclined stratum terminates down-dip against an initially sub-horizontal or inclines surface

Both onlap and downlap are indicators of non-depositional breaks (hiatuses).

Toplap

• Toplap is lapout at the upper boundary of a depositional sequence and is a form of offlap

• top lap is sometimes unrecognizable due to resolution limitations and the feature is mistakenly interpreted as truncation

indicators of non-depositional breaks (hiatuses).

Cedar Keys Formation

Cedar Keys Formation: dolostone and evaporites (gypsum and anhydrite) with minor limestone

Forms the base of the Floridian Cenozoic rocks over most of the state except NW Fl, where oldsmar formation forms the base

Middle Eocene Facies in FL

• oldest unit exposed at surface is the Avon Park Limestone (Middle Eocene)

• it occurs subsurface throughout the Florida peninsula and the eastern panhandle

Only locally exposed in sinks and quarries near the crest of the ocala platform in citrus and levy counties

The Avon Park Fm

composed of cream to light-brown or tan fossiliferous marine limestone (grainstone, packstone, and wackestone, with rare mudstone) interbedded with dolostone and gypsum

Ocala Formation

The Ocala Limestone typically consists of white or tan, homogeneous, porous and perme-
able, thickly bedded, foraminiferal limestone containing abundant granule to pebble sized
foraminifera, echinoids, mollusks, corals, and bryozoans. The Ocala Limestone characteristically consists of upper and lower lithozones (modified from Applin and Applin, 1944) which differ only slightly in average grain size and minor dolomite content

Suwannee Formation

Hawthorn Group –Coosawatchie Formation

Cypresshead Formation

Anastasia Formation

Undifferentiated Quaternary Sediments

Trail Ridge